Baja Music Man guitar and bass onboard preamps  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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Post by JayGunn »

THANKS ANDY! (sorry for shouting). What a difference. Fifteen seconds to fix it. Many thanks.

Thus ends the derailing of this thread for which I apologize.

John

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Post by nezvers »

Hi guys, I want to upgrade my friends Tanglewood bass guitar, but a thing I know only how to solder or how to do some measurements (not talking about that simulation thingy).
I'm a bit frustrated after going through this topic.
Can someone post which circuit is final/ latest version and what need to change according to pickup impedance? Asking because more I read than more it all looked like jibberish to me and I totally lost trace.

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Post by JayGunn »

Since this particular preamp is quite sensitive to the characteristics of the pickup, and since the MM pickup is unlike most other commercial pickups in having lower impedance, then it doesn't seem like a good choice for you to retrofit a non-MM pickup bass with this preamp. There are plenty of (made in China of course) 2- and 3-band preamps on eBay and from various guitar parts vendors that would probably work in a more predictable manner than putting in a 2-band MM preamp, unless your Tanglewood has an MM pickup(?) If it does, and you really want a soldering project, then just go for the original schematic that Bajaman posted on page 1 of this thread.

This preamp:
http://www.bestbassgear.com/seymour-dun ... preamp.htm
has switchable treble boost point so you can modify the sound to fit the pickup impedance, which has a big effect in many schematics on placing the pickup's natural treble resonance. Just once choice of MANY.

Hope this helps. I'd suggest not jumping into this as a DIY unless you're really eager to study and learn.

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Post by deltafred »

I used this preamp (with no mods) with a generic pickup and it sounds fine, just saying.

Post #8 has the correct schematic.
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Post by DavidRavenMoon »

I've used this preamp with standard impedance pickups (about 8k) and it rolls off a lot of top end. You can crank the treble, but the bass was never as bright until I removed the preamp. Passive it's much brighter. So unless you like a darker tone, this isn't suitable for standard style pickups.
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Post by PaulBass »

the op amp you use also plays a role in the frequency response. I notice most people use the stock LM4250 which I found to be dull sounding. I put this build in several basses using more modern op amps with great results. your MMV because there are so many different pickups and everybody's ears are different.

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Post by deltafred »

PaulBass wrote:the op amp you use also plays a role in the frequency response.
The LM4250 is a programmable opamp, from the datasheet "A single external master bias current setting resistor programs the input bias current, input offset current, quiescent power consumption, slew rate, input noise, and the gain-bandwidth product."

In the stock preamp it is optimised for quiescent power consumption (long battery life) at the expense of slew rate/gain-bandwidth product (frequency response). For a higher frequency response reduce the value of the 1M5 resistor between pin 8 and ground.

The original MM basses had the battery connected to the preamp at all times, battery life was still in excess of a year because of the extremely low battery drain (44uA).
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Post by JayGunn »

That's true about the 4250 op amp, but I don't think in battery saving mode that it cuts off the frequency response below 10-12K. Yes slew rate and GBW are reduced with the quiescent current.

But the real issue is that the MM preamp circuit is very sensitive to the impedance of the pickup. So for example (in simulation but I believe it) if the pickup impedance is 0.7h, like an MM pickup might be, and you turn the treble up to .99 there's a nice broad treble boost peaking about 4.5K. Now if you take the pickup impedance up to 8h, like an overwound P pickup could have, the treble peak drops to 1.5k, which you probably don't want, and the output has a negative spike of -40dB(!) at around 5200hZ. This would sound really odd and not like a treble boost at all. In fact that negative spike is also there when the controls are set flat.

(My) belief is that the circuit is so sensitive to the characteristics of the pickup that you'd just have to try it to see if the treble is useful for you. Or find out your pickup's characteristics and simulate it. But much easier to buy a Seymour Duncan, or 'Tone Monster' or 'Belcat,' or whatever.

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Post by DavidRavenMoon »

I increased the current draw on the preamps I make via the programming pin. Changed the resistor. The slew rate is better now and I think it sounds better.

But if you want it to sound like a vintage MM preamp you need to use that op amp. When combined with that bright pickup you get that tone.

More modern op amps, like a TL series have jfet input stages and a higher impedance. So they will sound a bit brighter. But look at the input impedance setting resistor. If you want a more transparent modern preamp, design something new with different a tone circuit. This is like a bass cut circuit and almost the presence control from an amp.
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Post by Andy_ »

We really should not dare to call the pickup a bright pickup because it is what it is ;-)

... and that is a low output, low impedance humbucker with coils wired in parallel to reduce dc resistance an inductance ...

It might sound brighter than other pickups when connected to *the* preamp but the reason is the treble resistor (respectively input resistors in the 3-band preamp) and the low resistance that leads to a higher peak.

Standard pickups can be used with the preamp and you can archieve nice results concerning the following:

[*]If possible rewire the pickups to parallel or make them switchable.
[*] alter the treble cap (lowering the capacitance)

The sound will be different, if you have more than one pickup connected to a selector switch you can really hear the changing treble peak when changing from one pickup to the parallel wiring. You will also note the reduced boost amounts because of the higher dc resistance.

I think Leo was just very, very clever: he minimized production cost of the pickup, compensated the missing output with the preamp and improved tonal possibilities. Placing such pickup in *the* position will do the magic, regardless of the body's or neck's wood, finish etc.

Btw.: If somebody would wind a very low output pickup (about 60%-70% of the current number of windings) the treble frequency would raise into hifi-sound-territory with increased gain. The frequency roll of of the LM4250 is still negigible in that range (I tried with a partialle unwound pickup as written above).

Regards
Andy

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Post by DavidRavenMoon »

Andy_ wrote:We really should not dare to call the pickup a bright pickup because it is what it is ;-)

... and that is a low output, low impedance humbucker with coils wired in parallel to reduce dc resistance an inductance ...
Technically it's not low impedance. It's medium impedance. 100 ohms would be low impedance. Maybe up to 1k.

The other thing is it's not at all wound like a low/medium z pickup. It's a fairly standard pickup wired in parallel. That reduces the resistance. But you still have all those windings.

Lower output pickups have a wider frequency response with the resonant peak being higher. On true low z pickups it's well at the top of the audio range.

I was making low z pickups some years back. They were about 1k with the two coils in series.
It might sound brighter than other pickups when connected to *the* preamp but the reason is the treble resistor (respectively input resistors in the 3-band preamp) and the low resistance that leads to a higher peak.
Which preamp are you referring to? This is the original 2-band preamp. And the tone controls are unusual. The bass control is just bypassing a bass cut cap. The treble control is bleeding highs through a cap from the output of the op amp. So it's negative feedback since it's inverting. This is similar t a presence control on an amp.

The reason why higher resistance pickups will sound darker is the input impedance of this preamp is too low. I'm posting from my phone, so I don't have the schematic in front of me, but I remember it being like 200k or something. Maybe lower. That loads the pickup down. This has the effect of lowering the resonant peak. The high frequency response of a pickup drops off right after the peak. So it's reducing the high frequency context and reinforcing the lows.

It sounds fine unless you want a bright tone. Then you need to either switch the pickups in parallel or go with pickups with less winding.

After having one of these in my '87 Ibanez with a set of my Neo Jazzbuckers (neodymium powered dual rail humbuckers in J shells... about 7k in series) for two years, I removed it and wired the bass passive. The bass is now much brighter. At some point I'll make a different preamp for it.
Standard pickups can be used with the preamp and you can archieve nice results concerning the following:

[*]If possible rewire the pickups to parallel or make them switchable.
[*] alter the treble cap (lowering the capacitance)

The sound will be different, if you have more than one pickup connected to a selector switch you can really hear the changing treble peak when changing from one pickup to the parallel wiring. You will also note the reduced boost amounts because of the higher dc resistance.

I think Leo was just very, very clever: he minimized production cost of the pickup, compensated the missing output with the preamp and improved tonal possibilities. Placing such pickup in *the* position will do the magic, regardless of the body's or neck's wood, finish etc.
No, Leo wound that pickup like that for the tone, not to save time winding! He made many prototypes for all his pickups.

And look at G&L pickups. They are not low output.
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Post by Andy_ »

...oops, maybe I messed up some terms (sorry, you might have noticed that I'm no native speaker/writer...).
DavidRavenMoon wrote: Technically it's not low impedance. It's medium impedance. 100 ohms would be low impedance. Maybe up to 1k.
The other thing is it's not at all wound like a low/medium z pickup. It's a fairly standard pickup wired in parallel. That reduces the resistance. But you still have all those windings.
Lower output pickups have a wider frequency response with the resonant peak being higher. On true low z pickups it's well at the top of the audio range.
I was making low z pickups some years back. They were about 1k with the two coils in series.
You're absolutely right, I meant "low resistance and low inductance regarding a passive pickup". Maybe we just can call it "medium output" - better than messing around with the frequency dependant "impedance" :D
DavidRavenMoon wrote: Which preamp are you referring to? This is the original 2-band preamp. And the tone controls are unusual. The bass control is just bypassing a bass cut cap. The treble control is bleeding highs through a cap from the output of the op amp. So it's negative feedback since it's inverting. This is similar t a presence control on an amp.
In this case you are wrong. The original 2-band preamp's treble control serves two things. It is ok to say that it cuts highs when turned back but the *magic* is the series resonance circuit consiting of the pickup's inductance, resistance and the input cap. Turning up treble will introduce the typical high mid peak (ok, bass players might call this "treble" :D)

DavidRavenMoon wrote: The reason why higher resistance pickups will sound darker is the input impedance of this preamp is too low. I'm posting from my phone, so I don't have the schematic in front of me, but I remember it being like 200k or something. Maybe lower. That loads the pickup down. This has the effect of lowering the resonant peak. The high frequency response of a pickup drops off right after the peak. So it's reducing the high frequency context and reinforcing the lows.
OK, I understand what you mean but... the frequency gain resulting to the resonance peak of the circuit shoud be much more audible (I'm not 100% sure but might do some tests...).
DavidRavenMoon wrote: It sounds fine unless you want a bright tone. Then you need to either switch the pickups in parallel or go with pickups with less winding.
That's what I meant: less winding means less inductance means shifting up the treble peak frequency means brighter tone. Or just use more pickups than two and wire them all in parallel...
No, Leo wound that pickup like that for the tone, not to save time winding! He made many prototypes for all his pickups.
Leo was a clever salesman... We will never know what really happened :wink:
And look at G&L pickups. They are not low output.
Do these basses sound like the original stingrays? I never player one, so I cannot say.

Regards
Andy

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Post by DavidRavenMoon »

Andy_ wrote:
DavidRavenMoon wrote: Which preamp are you referring to? This is the original 2-band preamp. And the tone controls are unusual. The bass control is just bypassing a bass cut cap. The treble control is bleeding highs through a cap from the output of the op amp. So it's negative feedback since it's inverting. This is similar t a presence control on an amp.
In this case you are wrong. The original 2-band preamp's treble control serves two things. It is ok to say that it cuts highs when turned back but the *magic* is the series resonance circuit consiting of the pickup's inductance, resistance and the input cap. Turning up treble will introduce the typical high mid peak (ok, bass players might call this "treble" :D)
First off, you said "three band preamp" which is why I said what preamp are you referring to? You didn't answer that.

That's not how it works. This is almost a standard op amp feedback tone circuit. Only it's set up like a T filter with the two different caps.

http://www.ti.com/lit/an/sloa042/sloa042.pdf

Keep in mind that caps will pass high frequencies and block lower frequencies, depending on the value of the cap (since they block DC, which can be thought of as a low frequency). The pickup is fairly isolated from the rest of the circuit. You can prove this by using a higher impedance pickup, or a pickup with a an buffer before the MM preamp. The treble control will work the same. The value of the caps determines the frequency.

Look at the impedance in parts of the circuit, and then see why it's not forming a resonant circuit with the pickup. Just as with passive tone controls, at the resonant frequency of the pickup, the cap looks more like a resistor, due to the impedance. And it's in line with the 1M pot. So to the pickup, it sees the 220k input resistance, and that's about it.

As I was saying earlier, the input resistor is 200k, so that's the input impedance of the preamp. That's much too low for standard high impedance pickups, insofar as you will have an audible loading effect. Generally you want about 1M.

The input cap is 1µF, which while it could be larger for a bass signal, mostly passes the signal unscathed. 5 to 10µF would allow more low frequencies. But since the whole circuit is lower impedance you can get away wth it. But it is probably limiting some of the very low end of the pickup.

The input cap is just there to block DC, same with the output cap.

The bass and treble controls are similar, both in the feedback loop. The treble control is just executed in an odd manner. But turned one way does roll off some high end. It's similar to the tone control in something like a RAT fuzz. But the caps pass high frequencies which are then reintroduced to the input of the preamp. They come from the inverted output, so it's negative feedback, which creates gain.
DavidRavenMoon wrote: The reason why higher resistance pickups will sound darker is the input impedance of this preamp is too low. I'm posting from my phone, so I don't have the schematic in front of me, but I remember it being like 200k or something. Maybe lower. That loads the pickup down. This has the effect of lowering the resonant peak. The high frequency response of a pickup drops off right after the peak. So it's reducing the high frequency context and reinforcing the lows.
OK, I understand what you mean but... the frequency gain resulting to the resonance peak of the circuit should be much more audible (I'm not 100% sure but might do some tests...).
And what is the resonant peak of the circuit? If you are trying to simulate this circuit in software, along with the pickup, it wont give you proper results. So far no one has gotten a pickup modeled adequately in software. They have been trying for years over at the Pickup Maker's Forum. You can get an approximation, but little things like eddy correct loss, and the like are lost.

You can get this preamp pretty flat. The treble boost is tuned pretty high.
DavidRavenMoon wrote: It sounds fine unless you want a bright tone. Then you need to either switch the pickups in parallel or go with pickups with less winding.
That's what I meant: less winding means less inductance means shifting up the treble peak frequency means brighter tone. Or just use more pickups than two and wire them all in parallel...
Lower impedance pickups have a higher resonant peak, it's not the inductance. This pickup has the same inductance as if it were wired in series, since it has a lot of windings. Then you have the large alnico magnets, which raise the inductance since they are iron based.
No, Leo wound that pickup like that for the tone, not to save time winding! He made many prototypes for all his pickups.
Leo was a clever salesman... We will never know what really happened :wink:
And look at G&L pickups. They are not low output.
Do these basses sound like the original stingrays? I never player one, so I cannot say.

Regards
Andy
I don't think you can decide what Leo was after... All the stories told is he loved making pickups. If you go to the G&L website, you can see his pickup room. He made countless variations of all kinds of pickups. While he did design things to be easy to manufacture, like bolt on necks, and the basic pickup contraction, he did not wind them to be easy to make. Why else would the P bass have 10,000 turns of wire on each coil. He did it for the tone.

Image

The G&L MFD pickups are interesting. They use large steel threaded inserts with adjustable steel poles, and large ceramic magnets. Then he has a steel plate under the pickup to increase the inductance (ceramic magnets have the inductance of air). The results are a very loud, and bright pickup. The G&L basses do have a preamp, but it's not needed to operate them. It's more like a boost circuit.

Here's an interesting post I saved one day from someone that worked at MM:

"Barry Klein:
You guys talk about the preamp and who designed it. I worked at Music Man in the 70's as Leo Fender's and Tom Walker's electronic technician. I worked in both the Music Man and CLF facilities. Leo was using an LM4250 op amp in a way it was not really intended for - but it did the job. He designed it by plugging in parts and strumming the 6 strings of his test bed neck. He spend DAYS doing that, driving everyone nuts.
It was never portrayed to me that Tom did the original preamp designs - but it would make sense as I could not believe Leo was designing with an op amp. But I suspect that Tom designed the ORIGINAL preamp, as since I started with Leo who was the one tweaking the guitar/bass preamps - not Tom. The use of the LM4250 for that preamp was kind of mindblowing too - everyone thought it was junk and not worthy of an instrument preamp. And God - he was using tantalum capacitors in the audio circuit! :-)
Leo knew that circuit better than anyone, knew what component did what and tweaked that damn circuit until both Tom and I just about went nuts. Tom sent me over there to work with Leo to try and finalize a design one time. I tried to suggest a totally different circuit. He didn't like it much. Leo didn't hear too well (after years of playing "ding dong ding" full blast in his closed lab) and he boosted treble to really high levels. Leo wasn't very current at that time. That is why the Strat is so bright and the stingray guitars ungodly bright. I would not get all religious with the LM4250 op amp that Leo used. It has really lousy specs other than supply current. the op amp was replaced with an LF441 later on - which has much better specs. Maybe there is even something better available now... I haven't kept track of op amp improvements. I would think that the boost in slew rate with the LF441 would result in a better sound with less distortion artifacts. Even when we were just starting out guys would replace the opamp circuit with a JFET circuit that someone was selling at the time."
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Post by Andy_ »

I think I'll stop quoting here to prevent the thread from exploding ;-)

Both preamps (2-band and 3-band) have some kind of feature in common and this is the 1.8n cap in the 2-band and the 8n2/10k in the 3-band. Both form a resonance circuit in combination with the pickup's inductance and resistance which both are affected by the number of windings.

I'd suggest to do a simulation of this part of the circuit with some typical MM-pickup parameters and sweep every parameter to see the effects.

If you do not believe in the simulation than I'd invite you to do a test like I did with the 2-band-design: connect different pickups to the circuit. You will than immediately hear that the simulation is correct (although in may be using a simplified model but this model does contain the essential parts).

What you will notice when turning up treble is that the treble peak frequency is affected by the inductance (a simple test is to use a two pickup bass and switch from one coil to both in parallel with treble maxed). You will also notice that the maximum amount of boost is reduced by a pickup with a higher resistance because the coil resistance is the part dampening that circuit.

Regard the treble pot as two pots in one: the left side undampens the treble peak and the right side adds a high cutoff (which is really "standard").

The 3-band design's "part in between the two amplifying stages" really is nothing special (you can find this circuit in many datasheets) but one (maybe it's *the* one) characteristic of the stingray it's enormous treble boost capability (regarding the 70s when it was designed). The 3-band takes this into account with the 8n2/10k right after the input.

Please look at this simulation:
Image

You can see the frequence response right after the 1st op stage, the pickup's inductance it swept without any eq following. The simulation ignores the raising dc resistance - so the maximum boost will be reduced with raising inductance (but a real test will make this audible).

The two band design shows a similar behaviour (treble pot max, inductance swept).
Image

And here is a simulation of the treble knob:
Image

My builds prooved that the simultion is generally true, although I could not make exact measurings which would require a device to alter the magnetic field with small spikes (btw.: knocking in the polepieces with the tip of a magnetic screwdriver and watching the resulting fft while turning the knobs can give us an idea...).

When I first build that pickup (btw.: thanks again bajaman!) I installed it in a two pickup bass that had a blend pot. The result was frustrating because the additional resistance of the pot immediately affected the treble response dramatically and made me start thinking, simulating and building and testing...

Regards
Andy

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Post by DavidRavenMoon »

You keep talking about the inductance of the pickup. How did you measure the inductance of the pickup? Did you actually measure a pickup? I'll tell you right now that there's pretty much only one LCR meter that can measure the inductance of pickups. And this is because of the frequencies used when doing the measurement. This is pretty much why most pickup makers ignore inductance. It's not that important for musical instrument pickups. Neither is DC resistance, but that's an easy measurement to make.

I think while the preamp model is fine, modeling circuits isn't that hard, you are making some incorrect assumptions about the pickup and its interaction. Pickups are not inductors, although they have an inductance. This is the problem with the current pickup modes being used. They are a bunch of parallel resistance/inductance/capacitance bits cobbled together. But real pickups are complex impedance devices. They do not work like a standard inductor in a circuit. This is partly because they are also generating current. So technically they are a variable reluctance transducer. In a preamp circuit, the DC resistance (and AC resistance/impedance) are more important factors. Measuring the impedance is tricky, since it changes with frequency.

The main effect you will find is that the input impedance of the preamp will shape the upper frequency resource of the pickup. I can even demonstrate this with my USB audio interface. The hi-z input sounds a bit less "warm" than the line/mic input, since the latter has a lower input impedance. This has the net effect of lowering, and flattening, the resonant peak of the pickup.

So, do pickups interact with preamps? Sure, insofar as the preamp will alter the tone of the pickup. On the MM preamp, and most other preamps of this type, the treble control is a negative feedback gain circuit. In actual listening tests, I found that the pickup yep doesn't matter that much. When the treble control is cranked, it sounds like a MM preamp. Even with totally different pickups, including active (buffered) models.

And of course the reason why a higher resistance (impedance) pickup wound sound different, including the treble peak, is that the preamp is loading the pickup down. But the resonance you see is from the feedback loop in the tone control. It has a higher Q than something like a Baxandall. I'd say any interaction was accidental, and not part of the design.

If you want an accurate plot, wire up a real pickup and preamp, along with the patch cord to the amp, and the amp's input impedance, and then use an exciter coil in introduce a swept frequency into one pickup coil (or both if you turn the coil on it's side, since they are out of phase). Then take a plot.
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Post by Andy_ »

DavidRavenMoon wrote:You keep talking about the inductance of the pickup. How did you measure the inductance of the pickup? Did you actually measure a pickup? I'll tell you right now that there's pretty much only one LCR meter that can measure the inductance of pickups. And this is because of the frequencies used when doing the measurement. This is pretty much why most pickup makers ignore inductance.
I would not dare to use any LCR-Meter :wink: And as I'm aware, that the pickup model is not complete it is still sufficient for me because the simulation's result are what is found in the real circuit, although there are slight deviations but I think it absolutely ok to have an idea of the range the values are in.
DavidRavenMoon wrote: It's not that important for musical instrument pickups. Neither is DC resistance, but that's an easy measurement to make.
We have a problem here... You are not willing to see the resonace circuit in front of the 1st op stage and I am not willing to ignore it. Maybe we better stop soon :cry:

Or maybe you could just try to connect some caps (500p, 1n, 2n) to the pickup and then change just listen to the result. You will immediately hear the changing frequency of the resonance peak. Then change the pickup to another with higher dc resistance and inductance... Another way to find about the range (I don't say exact value) of the inductance is to use a current (!) source (or a 10Meg resistor in series with a voltage source), a series cap and measure the pickup voltage. It's not absolutely correct but gives an idea after recalculation.
DavidRavenMoon wrote: So, do pickups interact with preamps? Sure, insofar as the preamp will alter the tone of the pickup. On the MM preamp, and most other preamps of this type, the treble control is a negative feedback gain circuit. In actual listening tests, I found that the pickup yep doesn't matter that much. When the treble control is cranked, it sounds like a MM preamp. Even with totally different pickups, including active (buffered) models.
I found the exact opposite. We took several pickups, did the test described above and then also listened after the measurement.
DavidRavenMoon wrote: But the resonance you see is from the feedback loop in the tone control. It has a higher Q than something like a Baxandall. I'd say any interaction was accidental, and not part of the design.
The picture below (feedbackLoopOnly) is a modified simulation. The 1n8F-cap is disabled, the 500pF-cap is switched on and off. There is a slight bell caused by the pickup, the 500pF is acting as a high cut. I think that's what they wanted because it eliminated some hiss when turning treble up.
feedbackLoopOnly
feedbackLoopOnly
2.png (2.73 KiB) Viewed 4211 times
When disconnecting the 500pF and switching the 1n8F we see:
1n8Only
1n8Only
1.png (2.7 KiB) Viewed 4211 times
Of course, this is only a simulation but my tests with real instruments proved it to be in the correct range...

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ruiluna
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Post by ruiluna »

Hello, I'm sorry for using a translator. I noticed that in the schematic the 4n7 capacitor is in the negative of the input capacitor, but in the PCB circuit you are in the positive capacitor. I installed the circuit according to the board. It works, but I always have a background "vrumm" background. Can you please, and given that all the components have the correct values ​​help me to identify the problem? My thanks . Rui Luna

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PaulBass
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Post by PaulBass »

there isn't a 4n7 cap in the circuit. If you meant 47n it can be ceramic or film which is non-polar. the 1uf input cap has the positive facing the circuit side

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Post by DavidRavenMoon »

Hmmm. There's the 1n8 cap right inside past the input cap. That's part of the treble tone control. But that's on the positive side of the input cap.
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Post by PaulBass »

ruiluna wrote:Hello, I'm sorry for using a translator. I noticed that in the schematic the 4n7 capacitor is in the negative of the input capacitor, but in the PCB circuit you are in the positive capacitor. I installed the circuit according to the board. It works, but I always have a background "vrumm" background. Can you please, and given that all the components have the correct values ​​help me to identify the problem? My thanks . Rui Luna
The "vrumm" I'm assuming is a hum and you made the circuit on a breadboard. you made a mistake somewhere thats making it hum. you gotta go through the circuit with a MM to figure out where you went wrong

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