freestompboxes.org

Here's my version of Bajaman's EVH overdrive, I call it the mini EVH. A friend brought his interesting circuit, which is directly inspired by the 5150 preamp, to my attention and I wanted to give it my own twist.
To Bajaman: know that in no way I'm saying that my version is better or there's anything wrong with its inspiration. They both sound great, I think of it as two different means to achieve more or less the same goal. I hope you're happy one of your circuits is getting “the treatment” Let's go through it:

• Removed the JFET input buffer. JFETs are expensive these days, and the 074 can do the job just as well. Input impedance is 1MΩ, input cap plenty large, input RF filter and current limiting have been scaled down for lower thermal noise while still avoiding pickup interaction. 68k was a common value for valve grid stoppers.
• Input stage scaled for more E6, resulting in an error of maximum 0.2dB. Capacitors go to ground, resistors still go to Vref, because I like to save caps too

  

• Gain control network: impedances scaled down, removed the resistor in parallel to the gain pot, pot made log taper, removed input cap. The bias for the next stage was already coming partly through the gain pot, but JFET op amps make this acceptable. The other values have been tweaked to get a very very close response across all the pot travel.

  

• Here's my bold move: before hitting the LED, we have three stages. Of those, IC1b and IC2b have some frequency dependent gain characteristics, while IC2a features a gain of 2, but it's after a 0.68x attenuator with flat frequency response (the low-pass before IC2a cuts in the hundreds of kHz). Attenuation is before amplification, so headroom isn't affected, only noise. I thought i could make those three stages into two and so I did.

  

  At the same time I saved some components along the way (the 33k and 10n at the output of IC2a aren't doing anything that I know of) and re-scaled to my E6 taste.
• You thought that was a bold move? The thing I found most interesting about the baja EVH was the idea of separately clipping the two half waves in different stages, but I really wanted to make this a 4 op amp circuit if I could. So I went looking at what's in between the two clippers. There's an attenuating low pass filter with an attenuation by 11 and corner frequency of 17kHz or so. I thought I could ignore the latter part. Following it there's IC3a, with a gain of 13 and flat response (low-pass at 49k). Combining this with the attenuation before it, since the series resistance is the same 2.2k and the diodes never conduct at the same time (I think we can say that ), the transistor diode is getting a signal 1.18 times bigger than the output of IC2b. In the realm of dB and gains in the neighborhood of 70dB, that's not much, so here's my fudge and maybe something you can lay blame on if the clipping (not the frequency response!) isn't the same: I just put the diode together with the led. Boring, but it works and seems reasonable to me. I've used a 4148 here, use a transistor if you want, personal choice.
  Running some simulations both for dc transfer and with transient, the visible difference is that the positive peak is limited to 1.6V instead of 1.88V, 1.18 times less, which is obviously because it doesn't get that little bit of amplification back. Since this difference is pretty close to the forward voltage of a schottky diode, I decided to do just that, and here's the result in DC transfer and transient with a 3V 1K sine, which is realistic given all the previous gain, and shows both clipping thresholds. Using a green LED instead of these two is also a viable option, and leads to the same results.
  This isn't the only way to go about compensating it, but I followed the dc transfer curves, which compare my clipping setup with the one with the attenuator and op amp in between, as main guide for both the clipping treshold and the output amplitude.

  

  evhtransfer.png (10.26 KiB) Viewed 4826 times

  

• Going on, there's a “buffer” stage with 2x attenuation followed by 2x gain. This goes away. The JFET buffer goes away too, for two reasons: first, the output impedance driving the tone stack is in the worst case 2.2k, even ignoring the diodes, and it would be happy even with more. Second, unless I'm wrong, in the original amplifier, even considering some negative feedback, the tone stack is driven by a series 470K resistor. That's actually a bit nuts, given that we're told to drive tone stacks from low impedances. So who do I follow, Peavey or Baja? I went with Baja in giving the tone stack a low impedance. I went with a 25k pot for Mids because saving one cheap resistor makes me oh so happy, and taper is the same if not better. Of course if you have a 50k pot the parallel is the right thing to do.
• Lastly, I went for tone stack “defeat” instead of bypass. The result is the same, but this allows me to use the second pole to switch off the make up gain in the last stage when the tone stack isn't active, so that the output stage can double as buffer. I was conservative in choosing the make up gain to avoid unintentional clipping, but since the loss depends on frequency and settings it's still approximate. Volume pot is 10K log, allowing you to plug this into a power amp for your own solid state 5150, or effects return, giving low output impedance at all settings avoiding unwanted tapering effects from low impedance inputs.

Demo (I just had to try tapping, didn't I):

I had fun making this and I hope you'll enjoy it too. I wouldn't have gone through the effort if I thought the circuit didn't deserve the attention.
I'm planning for a trans-national comparison of the two soon

Didn't see this coming. Thank You!

I'm really enjoying your circuit covers. I would love to read your checklist, although I already know much of it: E6, remove any part that cannot say what's its job in 5 seconds, minimal value resistors, disregard the name of silicon junctions .
Oh, and many thumbs up for providing demos

The G wrote: ↑27 Aug 2021, 06:42 I'm really enjoying your circuit covers. I would love to read your checklist, although I already know much of it: E6, remove any part that cannot say what's its job in 5 seconds, minimal value resistors, disregard the name of silicon junctions .
Oh, and many thumbs up for providing demos

hah! circuit covers, never thought to call them like that
Roughly that's it, although the lower impedances are for the sake of lower johnson noise, and another important point is that it should try to sound the same! cheers

dylan159, i also really enjoy your circuit covers! i'd love to see you take on the nobels odr1, full of weird values and (maybe, imo) excessive filtering.

asbestosaurus wrote: ↑29 Aug 2021, 05:32 dylan159, i also really enjoy your circuit covers! i'd love to see you take on the nobels odr1, full of weird values and (maybe, imo) excessive filtering.

The odr1 is kinda interesting. maybe someday, if there's more I can do then just values.

what value for D4?

rcustoms wrote: ↑01 Sep 2021, 03:43 what value for D4?

any schottky you like, 1N60, 581x, Bat. or just swap the pair for a green led.

Hello dylan159, I'm a newbie here, I've just start to built your mini evh with your schematic and combining directly with a low power amp of TDA2030A, in one board. Grab them in a small cabinet, so it could be a practice amp for my son. I wish it will works well.

Hey Junkbox! The tda2030 can sound very good. I recommend you do a design with current feedback to increase the output impedance and let the speaker move a little on its own for that natural speaker response that we love.

here is a great article on the subject:
https://sound-au.com/articles/current-drive.htm

Thank for your attention and the link, I wish I could learn fast.
The gain is quite big, the amp ( and mini evh is connecting) is whistling loud. But Then I plug in a pickup (only, without the guitar), the whistle is gone away.
Project not finish yet, but I believe the junk amp ( I realy build this without new components except ICs) will sound ok.

Are you using a switching jack? Where the Tip connects to ground when no input is connected?

The sleeve not soldered yet. Input tip of jack is floating free.
So the feedback rising strong, Turn the gain at 10 o'clock, the feedback faded. But when a pick up connected, the feedback still whistling at 3 o'clock (75% gain). Maybe I gotta reduce it little bit.
How do you say ?

Here is the trace of the mini evh+ 2030.

I'm not talking about the sleeve. Look at all guitar amps, they will use a switching input jack that switches the Tip to ground when no instrument cable is inserted.

Then, the next thing to check is how the gain of the 2030 is set up.

Yup. But I don't use input jack yet. Plugging I meant I connect pickup cable directly to the input on pcb. Sory my bad english.

I created the layout as like as mini guitar amplifier with all potensio, input jack, headphones jack and switches on the front panel. The mini evh acts as preamp.

I was informed of this blog that made a vero layout of the circuit. As I'm writing it's still unverified, but it's not bad at all so give it a try if you want a readymade layout:
https://dirtboxlayouts.blogspot.com/202 ... i-evh.html

By the way I'm not a moderator or anything, but I think it's best to keep the conversation on topic and free of somewhat unrelated troubleshooting.

I am getting a lot of feedback whenever I turn up the volume or drive. Any suggestions?

Did you build this layout?
https://dirtboxlayouts.blogspot.com/202 ... i-evh.html
Because, as you can read in the comment, it has that problem, indeed.

Anyway, before to say something stupid, tell me before if is that the layout layout you used, I have an idea to solve that problem that help me with the Dr. Boogie. (Though, I can't guarantee it with this layout.)