BH 420 bass amp; Help making a pcb layout

chicago_mike · Post by **chicago_mike** » 21 May 2009, 21:46

Hey all.

Okay I have a schematic of the venerable AMP BH 420 bass amp. This is the father of the EDEN, SWR, Thunderfunk and a few others.

What I want to do is make a pcc layout, but I could use some help as this is my first real try. I am all for posting the layout here as well.

One of the images is a pcb "see thru" so someone with a little skill in this could create one.

I would like to change 2 things:

1. Change the enhance switch to a pot

2. Raise the freq. of the treble control

I think this would make a great project for us bass players and for home stuio cats as well.

For services rendered I can offer parts, icecream, snickers, cuban sluts...you know...the norm around here..

chicago_mike · Post by **chicago_mike** » 21 May 2009, 21:51

Sorry for the GIANT sizes...

also heres the PREAMP schematic..the first is obviously the power amp..im not interested in that..its too old.

chicago_mike · Post by **chicago_mike** » 25 Jun 2009, 11:56

Friendly bump and a real Chicago pizza to any and all who would like to help with this bad boy.

gus · Post by **gus** » 27 Jun 2009, 15:24

I would buy a big solid state power amp for the power amp section.

The preamp looks to be maybe what people might like about this amp.

LImiting
EQ
high and low outputs

Preamp treble adjust C17
Are the preamps C7, C21, C22 and C28 tantalum caps?
preamp Q1 might need to be selected, need to check the specs of the JFET and work out the limiting design.

D2,3,4,5 are part of the clipping detecting nice how the diodes allow four stages to be sensed for clipping

What parts do people like?

chicago_mike · Post by **chicago_mike** » 28 Jun 2009, 16:16

The whole preamp section, limiter, eq, tone balance.

This bass amp inspired the SWR, Eden, Thunderfunk crew.

I want to build just the preamp section. I do believe they're tantalums the ones, you mentioned.

Im going to have a go at eagle cad, but it does seem a little overwhelming.

I also have a pic of the layout I can upload. gimme a few.

chicago_mike · Post by **chicago_mike** » 28 Jun 2009, 16:23

By michael_stram

Duckman · Post by **Duckman** » 30 Jan 2012, 16:43

Interesting. I'm starting my own PCB.

Duckman · Post by **Duckman** » 14 Feb 2012, 05:14

Well, almost done, but have some notation problems:

"Note 3: All * items may be 1/2 impedance shown"

Those items are some pots, resistors and two caps... Should I understand that as the half of the nominal value? Maybe in resistors, but .... in caps??

Other one: two caps in the power section are labeled as "10 x 2" What does it mean? 20uF or two 10uF?

Sorry 'bout my noobness

but any help will be really appreciated.

lietuvis · Post by **lietuvis** » 14 Feb 2012, 11:43

Duckman wrote:Interesting. I'm starting my own PCB.

I have this picture if that helps to anyone

Duckman · Post by **Duckman** » 15 Feb 2012, 00:46

Great! Do you have some picture of the component side, to confirm those values mentioned above?

Mmmm... maybe I can follow the routing to see what I get, but just the resistors, not the caps.

ppluis0 · Post by **ppluis0** » 15 Feb 2012, 02:22

Duckman wrote:Well, almost done, but have some notation problems:

"Note 3: All * items may be 1/2 impedance shown"

Hi Duck,

The potentiometers marked with asterisks are only those belong the EQ section ?

Or there are asterisks in other places of the diagram that I can`t see ?

All these pots are conected in parallel at the (+) and (-) inputs of an opamp. With the wiper at center there is no gain and no attenuation of each band, and you obtain boost or cut at each end, same as we expect of any graphic EQ

Doesn`t matter if ALL of them are of 100 or 50 K, but the taper MUST be linear.

This is an interesting EQ design as each freq band is also tunable in frequency.

You are intended to reproduce all the features of this design ? (limiter, enhancer, XLR out, bi-amp out, clipping indicator) or just plan to use the neccesary functions and leave the rest ?

Muchos Kudos,
Josè

Duckman · Post by **Duckman** » 15 Feb 2012, 07:47

Hola, Pepe

Thanks for your advices!
Not just the EQ section pots; look around the tone balance pot (upper right): R57, 58, 59 (pot), 60, and 61 are marked, but C29 and 30 (.01 each) are also marked

hence my question. How should do I interpret that "half impedance" note?

ppluis0 wrote:You are intended to reproduce all the features of this design ? (limiter, enhancer, XLR out, bi-amp out, clipping indicator) or just plan to use the neccesary functions and leave the rest ?

I'm going for the whole 420 pre package. I finished (almost) my ExpressPCB version of the preamp section, but I want to be sure about the correct values.
Any way and as you said, isolate the EQ section for a stand alone unit it's a good idea, but, by now, not for me. I was checking the gyrator values with the Muzique's calculator that you can find here http://www.muzique.com/lab/gyrator.htm and maybe I'll do some adjustements in the frequency ranges.

Saludos!

ppluis0 · Post by **ppluis0** » 15 Feb 2012, 18:55

Cómo estás, Pato ? (I´m sure that this forum will benefit if adopt more spanish words...

)

Perhaps the designer think that the parallel connected potentiometers belonging the EQ section can be all of 100 K or all of 50 K, but the componentes surround IC7B must be halved in value if were need to use half the pot value. I think that is a benefit at time to buy components.

After see the dimension of the board in the photograph posted yesterday, my best guess is that two decoupling caps at each supply (C38 and C39) must be placed somewhere in the rail traces to avoid any noise.

Muchos saludos,
José

Duckman · Post by **Duckman** » 15 Feb 2012, 21:33

Err

Ok, but... What about "1/2 impedance"? How this concept must be applied to caps? I'm lost about that!

Maybe I should use the stated values.

Abrazo.

ppluis0 · Post by **ppluis0** » 16 Feb 2012, 04:03

Buenas noches, (good night)

I imagine that in case of not to have 100K pots in stock, at time to solder, a batch of preamps fitted with 50K pots works equally well.

(There is also a break to the people at the purchase dept...

)

But the only position at which some components vary as a function of the pot value is near IC7 b

I`m agree with you in your perception to use the stated values. Also is reccomendable to place among the two pairs of 10uF electros, some 100nF caps from each supply rails to ground at several places of the pcb, due the great area needed by the board.

Perhaps the long supply tracks can act as antenna and electrolityc caps are not enough to shunt rf noise as little ceramic caps.

Salud (cheers)
Josè (Joe)

Duckman · Post by **Duckman** » 16 Feb 2012, 06:00

So, after googling about capacitors impedance

Capacitors are different from resistors and inductors in that the impedance is inversely proportional to the defining characteristic, i.e. capacitance.

That means I have to double the value in both .1 capacitors to .2 (.22 in fact) to obtain "1/2 impedance", right?

Any way, looking in detail the picture posted by lietuvis, you can see two 47k resistors near the Tone Balance pot (third from R to L) where R58 and R60 must be, wich is the value stated in the schemo, so I'm going to use these values and forget all that "1/2 impedance" shit

I'll post some pictures when finished.

Meanwhile, thanks ppluis, lietuvis and the rest of the bunch

ppluis0 · Post by **ppluis0** » 16 Feb 2012, 12:09

Duckman wrote:So, after googling about capacitors impedance
I'll post some pictures when finished.

Another legendary preamp to build is the Ampeg SVT, that has been reissued as rack unit alone (don´t remember now the exact model name), but employ a specially wounded inductor in the midrange EQ that I think is a difficult to reproduce to attempt a clone.

I look forward to see your layout finished

(I don´t know what this smilie means, but looks funny...)

Nos vemos luego (See you later...)
José

lietuvis · Post by **lietuvis** » 16 Feb 2012, 21:27

ppluis0 wrote:
Duckman wrote:So, after googling about capacitors impedance
I'll post some pictures when finished.
Another legendary preamp to build is the Ampeg SVT, that has been reissued as rack unit alone (don´t remember now the exact model name), but employ a specially wounded inductor in the midrange EQ that I think is a difficult to reproduce to attempt a clone.

I look forward to see your layout finished (I don´t know what this smilie means, but looks funny...)

Nos vemos luego (See you later...)
José

as for Ampeg midrange inductor you can get it from http://www.sowter.co.uk/. You can actually order inductor with any given specification, the only thing is the price quite high. I paid around £50 for two of them, I ordered 800mH with a taps 100mH and 300mH same as Ampeg. And to my ears they sound so sweet.
Here is my style breadbord to test it, never boxed it up because thinking to install it onboard on my bass.
BTW I used Ibanez eq schematic, but used to control only midrange which in my case all i needed.

Duckman · Post by **Duckman** » 16 Feb 2012, 21:29

In case somebody need it... I'm too bussy with the 420 and the CX3 Leslie sim

mozwell · Post by **mozwell** » 17 Feb 2012, 03:35

All
I built my own SVT-2P pre several years ago now, & made my own inductor
RM8 un gapped cores, if you can get some very fine enamle coated wire, you can wind your own.
Here is the one i made & the circuit i used.
You should use a fixed resistor instead of R35 & RV5, as there is DC on the pot (not the best) so its scratchy when it is turned. I normally leave it fully turned up.
I did a pcb layout which i hope you can use. A few others have made this from my pcb art.
If you use the pcb art, make sure you print it 1:1 from the pdf.