MuAmp Bluesdriver

[comfortably_numb]

Here's a quick draw up of something I'm thinking of doing.  Two mu-amp stages (SRPP really) with the tone networks from a Keeley modded bluesdriver.
Using the SRPP, I'm thinking this should be a fair amount heavier than the BD-2 

I'm not sure about a few things though.

1. Are the tone networks after each stage sufficient or do I need interstage/output caps?
2. The SRPP stages themselves are stolen from other schematics - BSIAB or something...can't remember, I looked at many.  What are the bypass caps doing for gain/frequency response?  I've looked at all the articles at GEO on mu-amps and two FET circuits but this is so basic I'm having a hard time coming up with anything in my searches.

Anyway, thanks for looking.  If anyone feels like breadboarding I'd be happy to hear about the results.  I tend to do a lot more designing than actual building.  I'll work up a layout as soon as I get some feedback too.

Thanks guys!

[Quackzed]

thats a cool idea
the bd-2 is a very transparent od/dist at low gain, mainly imho due to the tone stack/ network...
its got a good responce that doesnt get muddy, thhough they can be a bit fizzy at high gain.
still i think using mu-amps would be a good match for that type circuit. nice

[comfortably_numb]

The "keeley" mods do a good job of taming the harsh high end.  I really enjoy mine.  It has a really nice lower midrange.  I'd like it in a stable higher gain flavor.

[Ice-9]

I have had good results with a similar design, although mine has different interstage filtering and tone sections. The reason I built my version was because I bought a Boss BD2 and thought it sounded awful.

The schematic can be found here for comparison. http://www.diystompboxes.com/smfforum/index.php?topic=91136.0

[comfortably_numb]

I would expect a high gain from the 20k and 10k source resistors here.  By your description I must be mistaken?

[JDoyle]

On the interstage coupling you are fine. If you take a look at it, you have caps blocking DC from getting to the next stage (or the output) and upsetting the bias. If you can't make it from the output of one stage to the input of another without going through a cap, you have DC isolated both stages.

As far as the source resistor/cap combos it works just like a normal filter but in this case the frequency response of the simple filter (f=1/2piRC) is superimposed upon the gain of the stage. That is, at the frequency where the impedence of the cap is equal to the resistance, the gain is down -3dB from the max gain of the stage, below that freq. the gain is reduced proportionally (along a line with a -6dB slope), above that frequency the gain increases until the point where the cap can be considered a short to ground for AC - and the circuit is operating at full gain.

The above is a bit of a simplification - there is another -3dB point, though it rarely becomes in issue for the freq. we are dealing with - but in essense, that is how it works.

Regards,

Jay Doyle

[WGTP]

From the MuSrpp's on my breadboard, it appear that with the 1k source resistor a 4.7uf bypass cap provides full range response.  When it becomes smaller around 1uf it starts noticably rolling off the bass.  At .1uf it acts as a treble boost.  Use it to fine tune the frequency response. 

[Ice-9]

I would expect a high gain from the 20k and 10k source resistors here.  By your description I must be mistaken?

Yes your correct having 20k source resistor in the first stage would suggest quite a high gain, but as I have used a 22n bypass cap with it the gain is kept low by rolling off the low end but allowing the higher frequencies pass. In your circuit you have a 1K/1uf combination, the 1uf cap restores quite a bit of the gain .

[comfortably_numb]

Thanks for the replies, they were all quite helpful. 

Jay - I thought that would be sufficient to block DC between stages and output.  Thanks for the confirmation!

WGTP - Thanks for the bypass cap info!  This is good news.  Now I can fine tune both stages to match the opamp stages of the BD-2.  I also read a thread I believe you started about the frequency effects of the bootstrap cap, but it was rather complicated, and I don't think a real formula was ever posted.  Is there a general rule for where to start here?  How does it interact with the source resistor/bypass cap?  The original mini-booster used a 3.3uF, and later Jack stated that a 1uF would be just fine.  The BSIAB2 uses a tenth of that value in the second stage and only .068uF in the first.  That seems like an awfully wide range of values for this component.  And it's not addressed at all in the GEOFEX SRPP article (except being 0.1uF in the schematic).  Any insight?

Thanks again! 

[WGTP]

It sort of acts like the source bypass cap in that it appears to roll of the bass.  You will see various values used, 100n being the most popular.  IIRC I didn't notice any difference below 220n.