Please help me understand these resistor values. (Cloning a SS guitar amp) - Page 1
Forum section: electronics / beginners

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 06:18:56 pm
content: This appears to be a differential amp, but I can't understand why R3 is 6.8K instead of 5.1K to match R5. Is there some current or voltage offset that the designer needed to reduce, or was he intentionally introducing an offset? I couldn't figure out a reasonable explanation for that value using Ohm's Law. I also couldn't find this particular topology with the coupling capacitors outside of the Bronco and Bullet service manuals.

author: fcb
author_rank: Super Contributor
timestamp: November 18, 2020, 06:41:23 pm
content: You can look at thislargelyas standard non-inverting opamp configuration with R6+R5 being Rf (Rfeedback) and R3 being the Rtg (R to ground), this will give a gain of 1+((R5+R6)/R3)=3.96C2, R2 act as a high-pass filter.R4 will alter the filter gain at certain frequencies (perhaps giving a small peak/notch in the frequency response) - without modelling it or thinking (with more coffee) I couldn't say.

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 06:48:46 pm
content: This is the part of the Bronco amp that has me cloning it. The amp has a reputation for sounding "tubey" and this topology seems unique to these two amp series, so I'm suspecting this stage has a lot to do with its tone. (The rest of the schematic has no surprises - volume, tone, LED distortion all in feedback loops. Yawn.)I'm tempted to build up just the first two stages, whack a 10K pot in place of R3 and see how adjusting it affects a sine wave.

author: fcb
author_rank: Super Contributor
timestamp: November 18, 2020, 07:16:12 pm
content: Simulate it in LTSpice - if you haven't used it before the learning curve is not-that-bad, certainly a bit slower than building it (the learning curve that is).

author: Renate
author_rank: Super Contributor
timestamp: November 18, 2020, 07:21:09 pm
content: My take on this:As stated, it's basically a x4 amplifier.R4 is "bootstrapping" to increase the input impedance.It has to compensate for R2 (which is needed for DC bias).Well, why isn't R5 & R3 the same value?If it were then the input impedance would be infinite.They decided to hedge their bets and make the gain at R4/R5/R6 less than 2.Why did they bother doing all this?Because this is a guitar amplifier and they want tube-like input impedance.(You never did mention that this is a guitar amplifier.)

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 07:26:56 pm
content: Yes, I should have explicitly mentioned it's a guitar amp. Forgive my provincialism; I should realize most people won't automatically connect the name to Fender.

author: Renate
author_rank: Super Contributor
timestamp: November 18, 2020, 07:30:18 pm
content: Well, silly me. I read "Bronco" and I think of a pickup truck.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 18, 2020, 07:35:47 pm
content: I was more thinking of:https://en.wikipedia.org/wiki/Bronco_BillyAs it has only one input, it is definitely not a differential amplifier.As others have already written, It's an amplifier with some filtering around it. You can either copy the circuit as is, or put it through some spice program if you want to understand or modify the filter characteristics better.

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2020, 09:34:33 pm
content: Wouldn't it make more sense to simply increase the value of R2, rather than using bootstrapping, to increase the input impedance? The TL072 is a J-FET input op-amp, with low bias currents and a huge input impedance. R2 could easily be increased to 10M or more, especially if the output is also AC coupled and it isn't being operated at high temperatures, when the bias current is significant.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 18, 2020, 10:56:04 pm
content: C1, C2, R2 and R4 form a 2nd order Sallen-Key high-pass filter with an effective Q/damping set by the ratio of R5 and R3. R6 provides extra gain (approx 7dB) across the whole bandwidth. I suspect that R1 is there to provide some isolation from reactive inputs, and possibly provide some protection against over-voltage inputs.

author: pvsage
author_rank: Contributor
timestamp: November 19, 2020, 01:42:40 am
content: I apologize; it has come to my attention that my most recent reply may have come off as terse and unappreciative. I did intend it as an apology for my musician's provincialism. I also apologize for being a bit cagey about the kind of amplifier I'm attempting to clone; Fender is notoriously litigious about anything that smells like copyright or trademark infringement, even if it's a hobbyist attempting to recreate a product that is no longer available. (They usually claim it's about protecting their brand identity, but I'm sure it has a lot to do with the possibility of future reissues.) Oh well - "Meow! Cat's outta the bag."Renate, thank you so much for such an interesting and insightful analysis of the circuit! Since nothing else about the amplifier stands out, I was approaching this from the premise that the choice of resistor values was to produce some asymmetrical compression similar to that of a fairly warmly biased triode; your analysis suggests I'm on a wild goose chase down a rabbit hole.It's also curious to me that the Bullet amp hasn't met such acclaim as the Bronco, considering they have exactly the same circuit. Maybe they changed the speaker and the one in the Bronco was actually voiced more like purpose-made instrument speakers, or maybe it's because the Bullet is still part of a "starter pack" with a shaffordable Strat for under $200; maybe it will begin to become coveted only after it is retired as the Bronco was. (Maybe word just hasn't gotten around that the Bullet is actually a Bronco wearing a pleather jacket rather than a tweed one and guitarists have yet to realize the amp is no longer Unobtanium.)

author: pvsage
author_rank: Contributor
timestamp: November 19, 2020, 01:55:07 am
content: Andy Watson and fcb, thank you so much for pointing out the high-pass filter! I should have spotted that straight away; I've been out of the game too long. Let's see, that's about 66Hz. Maybe to filter out line noise picked up by the cable shield and single coil pickups? That also answers why they went with an R48 value. Thanks also to everyone who suggested simulating it; I'll work on that while waiting for the boards to analyze this in meatspace.Three standard Strat pickups in parallel are a very low impedance source. Maybe R1 is to reduce the load seen by the guitar?

author: Renate
author_rank: Super Contributor
timestamp: November 19, 2020, 02:39:31 pm
content: Well, thanks to Andy Watson for picking up on the Sallen-Key filter. I missed that.The lowest note (E2) on a guitar is 82.4 Hz.Since line hum 50/60 Hz on unbalanced audio lines is such a problem, it makes sense to slice it off at the input.Quote from: pvsage on November 19, 2020, 01:55:07 amMore likely just to protect the opamp from static or foreign voltage on a loose cable.I make the input impedance out to be 2.055 M.You can see that R1 only contributes 15 k of that.

author: pvsage
author_rank: Contributor
timestamp: November 19, 2020, 06:34:09 pm
content: Don't R1 and C1/C2 also form a first-order low-pass filter, maybe to keep out radio signals? I'm sure we've all heard anecdotes of guitar amps picking up ATC chatter.

author: Renate
author_rank: Super Contributor
timestamp: November 19, 2020, 06:50:30 pm
content: Nope, there's nothing limiting the high frequency but the op amp itself.There is another thing that affects the response.Although this is a Sallen-Key high pass filter, the feedback is not at unity, it's x 1.75I'm not sure exactly how that affects things.

author: pvsage
author_rank: Contributor
timestamp: November 20, 2020, 06:25:53 pm
content: This project has been rattling around in my head so many years, I have the patience to wait until I have it on copper to test. The layout for the first couple stages is so straightforward I could probably lash it up on perfboard if I lose patience.Working on the tonestack now. I've built valve amps before, but this solid state gear is a completely different rodeo! Routing the tonestack is proving more tricky than I expected.