Author Topic: Green Bean to Matsumin's SRV Special  (Read 123 times)

BrokeDownSouth

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Green Bean to Matsumin's SRV Special
« on: November 12, 2017, 11:09:25 AM »
I had a few questions about this build, but the first was the omission of R5 from the Green Bean schematic (10K between pin 3 of opamp and Vb).  What difference would not having the bias voltage on this spot make?

http://www.aronnelson.com/gallery/main.php/v/ulysses/ulysses_layouts/ulysses_ts_srv_special/matsumin_ts_srv_special_schem.gif.html

madbean

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Re: Green Bean to Matsumin's SRV Special
« Reply #1 on: November 12, 2017, 11:39:19 AM »
At first I thought maybe it was a bi-polar power supply in which case I guess the buffer and input of the gain stage could be directly DC-coupled, but that's not the case here.

Quoting from RG Keen's Technology of the TS:
Quote
The clipping stage is a fairly ordinary variable gain opamp stage with a few tricks to shape the amount of clipping and the frequency at which it occurs. The signal from the input buffer stage feeds the (+) input of the opamp section, so the output is in phase with the input. In all members of the TS family except the TS10, this is a direct connection from the coupling capacitor to the emitter of the input buffer. In the TS10 there is a 220 ohm resistor in series with the (+) input. In all models the input is biased to the 4.5V bias source with a single moderate value resistor, usually 10K; this seems not to make any difference in the sound.

But, I'm not sure if he's referring to the 220R resistor or 10k resistor "not making a difference to the sound". It seems to me a 10k referenced between the emmiter of Q1 and ground rather than to the bias voltage at the non-inverted input of the amp would make a very big difference and not sound good at all. But, I might be totally wrong about that.

It's easy to find out - just breadboard that input buffer and a fixed gain stage and see what happens.
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BrokeDownSouth

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Re: Green Bean to Matsumin's SRV Special
« Reply #2 on: November 12, 2017, 12:15:52 PM »
I had planned on breadboarding it, but I'm early in the build and was just going over the schematics.  I brought this up because I had built a couple 'Matsumin SRV's' on some solderable breadboard and then a while later I build one based on the Green Bean.  The Green Bean sounded a lot different.  I'm taking another look now after a several-year hiatus from pedal building; comparing the schematics in detail.

I was wondering if R5 is not needed due to the coupling caps being omitted - maybe pin 3 opamp input is getting DC voltage from the -> transitor because C2 is omitted in the Matsumin version.

Just trying to learn some theory, feel free to correct me.

BrokeDownSouth

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Re: Green Bean to Matsumin's SRV Special
« Reply #3 on: November 12, 2017, 12:23:15 PM »
Just noticed on both schematics pin 5 of the opamp (non-inverting again) have a 10K resistor to Vb (R9 on Green Bean).  I wonder, again, if it is not needed on pin 3 due to C2 being omitted.

reddesert

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Re: Green Bean to Matsumin's SRV Special
« Reply #4 on: November 14, 2017, 02:32:29 AM »
I was wondering if R5 is not needed due to the coupling caps being omitted - maybe pin 3 opamp input is getting DC voltage from the -> transitor because C2 is omitted in the Matsumin version.

I think this is correct.  For example, compare the snippets of the DC-coupled and AC-coupled non-inverting amplifiers at the top of page 2 in https://ocw.mit.edu/courses/media-arts-and-sciences/mas-836-sensor-technologies-for-interactive-environments-spring-2011/readings/MITMAS_836S11_read02_bias.pdf