GGG Brassmaster: Does anyone know the correct resistor values?

[MR COFFEE]

Hi all,
Just looking at the schematic, it seems likely to me that C12 might not be in the collector circuit of Q6 as shown, but belong in the base circuit so the bias doesn't go nuts, as in a coupling capacitor from the output of the Twin-T to the base of Q6. Make sense?

FWIW

[R.G.]

Hi all,
Just looking at the schematic, it seems likely to me that C12 might not be in the collector circuit of Q6 as shown, but belong in the base circuit so the bias doesn't go nuts, as in a coupling capacitor from the output of the Twin-T to the base of Q6. Make sense?

Kind of. Your premise that bias should not shift when switches are flipped is good, and that's what led me to note that C13 was shown incorrectly. However, C12 or some equivalent is needed for blocking the DC from the collector to the output; it may be that it ALSO needs another cap at the input to Q6, and that is one of the things I've been messing with. As soon as I get back from the holidays I'll have more info on that.

[DavidRavenMoon]

As soon as I get back from the holidays I'll have more info on that.

I'm going to call 2009 the Year of the Brassmaster!  Chris Squire will be proud!

BTW over at another forum someone posted a photo of the trace side of the PCB, as well as a PDF of the board traces after it had been run though some software, and the board with the parts layout.

The guy that did the layout said he took the "incorrect" schematic and redrew it into Circuitmaker and was able to verify that the schematic is in fact correct, and matches the real pcb. He also said the Q1 supply is not marked, but it should be fed from the 8 volt supply.

This is all very exciting for this old Brassmaster fan.   

I've always missed my BB and my Univox Unitron (Mutron clone) and my script Phase 90, and my Guild Foxy Lady, and my...   

[George Giblet]

> BTW over at another forum someone posted a photo of the trace side of the PCB, as

I've checked the following:

- Traced parts overlay pdf and compared with the *original* schematic.

http://files.muziq.be/schematics/maestro_bb-1_001.gif

- Checked wiring of pots and switches matches *original* schematic and photos.
- Traced parts overlay pdf with top side photos, appears to agree but I can't see all the value.

Haven't fully checked:
- Traced parts overlay pdf with bottom side photo
- Pot values

At this point it appears the original is correct.  So all the propaganda about the original schematic being wrong is crap!

Interestingly moosapotamus build one just like that,

http://www.moosapotamus.net/THINGS/brassmaster.htm

The sound samples don't sound like the b:assmaster samples, possible because of direct recording vs speaker or speaker emulation.

Note is Q5 is a darlington, and is therefore different to the other transistors.  On the original schematic, the top view pin out picture for Q5 is wrong (the schem itself is OK) - the 2N5308 datasheet has the same pin outs as the 2N3392.

One thing which would be nice to know is the DC resistances of the transformer windings
- primary:  center to outside1, center to outside 2 (need to measure with both meter polarities and take largest reading)
- secondary

[gigimarga]

I hope that you all will solve the mystery as soon as possible because i got the transistors and i will build one these days

[R.G.]

> BTW over at another forum someone posted a photo of the trace side of the PCB, as
I've checked the following:
- Traced parts overlay pdf and compared with the *original* schematic.
http://files.muziq.be/schematics/maestro_bb-1_001.gif
- Checked wiring of pots and switches matches *original* schematic and photos.
- Traced parts overlay pdf with top side photos, appears to agree but I can't see all the value.
Haven't fully checked:
- Traced parts overlay pdf with bottom side photo
- Pot values
At this point it appears the original is correct.  So all the propaganda about the original schematic being wrong is crap!

It's interesting. I found the photos of the top and bottom of the PCB as well, and did my own tracing from that, to the extent possible from the photos. I also took the copied layout and parts overlay and compared that to the original schemo as well.

Net result - while I can't see all the detail, it does appear that the factory schemo could well match the photos I've seen. I also managed to get a sim to run on the whole forward path. When I finally got the simulation failures stomped out, it did seem to work, at least somewhat.

I did find that the bias points of the buffer and filter amp vary with switch setting, as I said. One of the brass positions gets really dull instead of peaky because of that. The response curves get more interesting if the changes to C12 I mentioned are put in, and more importantly a new cap inserted between the T networks and the base of the filter amp. But those do not appear to be in the original.

... if the photo is of an original, or is of an original that has the sound that we hear in samples.

I'm still hamstrung by not seeing a real one ever. And I'm getting paranoid  () about photos as well as schematics.

Anyway, one day I'll find a real one and this will get solved. Shoot, the first layout I did may have accidentally been accurate, if unsatisfying.  

[George Giblet]

I still hold some skepticism but after checking through those fairly clear photos it's diminishing rapidly  .

> I did find that the bias points of the buffer and filter amp vary with switch setting, as I said.

Yes they definitely do. 

> ... But those do not appear to be in the original.

In the photo I checked that part of the circuit many times and it appears it's just like that.

With a darlington for Q5 the bias points are very poor, particular with the filter set to the 6k8/3k3 position.  I'm seriously thinking all the transistors are alike.  When Q5 is a normal (non darlington) BJT the bias points aren't too bad at all.

[R.G.]

I've thought a few times about reducing that to modern (i.e. opamp) practice, and that would fix things.

See what happens in your sims when you move the C13 cap to the base side and insert a 1uF in series from the Twin T to the base of the filter amp. Now there is no more bias shift. The filter resonance give two nicely distinct peaks at about 700 and 2.5K. I thought for sure that was what was in the original, but it appears not.

[DavidRavenMoon]

At this point it appears the original is correct.  So all the propaganda about the original schematic being wrong is crap!

Interestingly moosapotamus build one just like that,

http://www.moosapotamus.net/THINGS/brassmaster.htm

The sound samples don't sound like the b:assmaster samples, possible because of direct recording vs speaker or speaker emulation.

He also says that the Brass switch doesn't really work.  The only difference between the original schematic and RG's is the twin T filter, and they are functionally the same.

I built the same clone and the brass switch doesn't work. It sounds just like the one in the samples.  It doesn't sound bad, just different.

The problem seems to be that in the original schematic, Q1 does not have it's supply voltage listed, so the assumption is it is 9V, but the guy that reverse engineered it at the other forum says it should be 8V, and that's the only mistake.  He says he has a pedal built from RG's layout and with that fix it works.

I'm going to get the details from him and give it a try after the New Year.

[R.G.]

The problem seems to be that in the original schematic, Q1 does not have it's supply voltage listed, so the assumption is it is 9V, but the guy that reverse engineered it at the other forum says it should be 8V, and that's the only mistake.  He says he has a pedal built from RG's layout and with that fix it works.

I went back to my friend the circuit simulator.
(Here is the standard disclaimer for the circuit sim nay-sayers: yes, yes, I know that circuit simulators often lie. I'm familiar with the ways they lie, and some ways to avoid them. They are almost always deadly effective on things like DC bias and low frequency gain, though. Like I'm using it here.)
The input stage changes its collector bias voltage by 90mV when the supply voltage is changed from 8 to 9V. The forward gain at the collector changes from 21.68db to 21.81db in going from 8V to 9V and the "gain" at the emitter changes from -1.073 to -1.09db with an input of 100mV peak sine wave at 600Hz. The gain is substantially flat from 120Hz up to over 100kHz.

That is to say - except for the minor change in clipping voltage from the smaller supply voltage, it doesn't change in any noticeable way at all.

The change I suggested, moving C13 to the Q7 base side of R29 makes a much bigger difference in the sound, according to what you said, David, and in the direction of the way you remember it sounding. But the PCB info I have says it's not that way. The other change I suggested that's also not there, putting in a 1uF cap between the base of Q6 and the filter circuit makes a change that will be clearly audible too.

[George Giblet]

See what happens in your sims when you move the C13 cap to the base side and insert a 1uF in series from the Twin T to the

If I keep Q5 (the filter-amp transistor) as a normal transistor I'm getting good bandpass filtering behaviour with the circuit *as is*.  If I use a darlington the bias point  almost turns Q5 off which is rubbish.  With the non-shifting bias point mods I get the similar results.  From a technical point of view I'm all for non-shifting bias points.  The shifting bias points cause nasty "pops" when the switches are changed. 
(BTW on the original circuit the filter-amp is Q5 on the geofex marking this has been changed to Q6.)

As far building an original clone we shouldn't have to mod the circuit, it should sound correct despite the technical issues.

The question is why doesn't it sound right!  There's two assumptions which might be wrong:

- The transformer.  We are assuming it's a 1:1 transformer.  Maybe it's not, maybe the output side produces a hotter signal that would make the filtered signal and the harmonics switch more responsive.
- ceramic cap distortion.
- The filter switch wiring "looks" like it matches the original schematic, however, some switches have "funny" connections.  IMHO, this is a possibility but an unlikely one.

The problem seems to be that in the original schematic, Q1 does not have it's supply voltage listed, so the assumption is it is 9V, but the guy that reverse engineered it at the other forum says it should be 8V, and that's the only mistake.  He says he has a pedal built from RG's layout and with that fix it works.

The change from 8V to 9V is going have a minor effect.

It's frustrating when everything looks correct but it doesn't sound correct!!!

[George Giblet]

Another assumption is the pot values.

If VR3 was higher, say 100k, the bias shifts change and the output of the filter would be hotter.  This would have a noticeable effect on the sound.  Also if this was done I'm not sure if Q5 should then be a darlington.

[DavidRavenMoon]

OK, this is where it gets weird... I just got done talking to the guy that posted the circuit board trace at the other forum. He's also the one that said Q1 is supposed to have the +8V supply.

He says that the schematic is correct, and that the Q1 issue is not that important.... notice that the B: Assmaster clone only uses 9V. He ran the schematic and the trace from the original board though circuit checking software, and that's where he found the Q1 issue, but everything else checks out.

He also went though and took readings on the carbon comp resistors in various parts of the circuit and found they were higher than stated, but he didn't think that was all that important, except that some Brassmasters he tried sounded better than others.  He also thinks the diodes might have been matched.

What he thinks is going on is just the actual layout of the original, with the myriad wires, and all those ceramic caps close together.  He says he modded a clone to sound like the original and also compared it on a scope to the real thing.  Apparently it was done by using the original style parts and tweaking some values.

So based on this info he thinks the schematic is correct, it was a sloppy circuit design, and just happened to work well in the context of the original pedal.

He lives close to me so at some point I'm going to check out his clone.

So RG, unless you learn something new from Malekko, this is still a mystery.  I'm curious what he thinks the change is.

[gtrgeek1]

Hi everyone. I'm the guy that did the copy of the original pcb layout. When I couldn't get a great sound out of my clone, a friend lent me his original. I was convinced that the schematic was wrong at the time, and that the problem was in the filter section. After the schematic and layout were input into the Traxmaker, the computer kept giving me an error between the two. When I went back to check against the schematic it was clear that the only error was Q1s supply. This really didn't make any difference in the tone of the unit or the function of the switches. I just wanted people to know that the schematic on the internet matched a brassmaster that I had in my possesion. It is entirely possible that there are other units revisions with circuit variations.

I modified my board with the 8 v supply on Q1, changed all the caps in the filter section to ceramic disc (as per original), and changed the resistor values to the actual values of the drifted parts in the original and it was right there. Here are those resistor values - 47k measured 54k (use 56k), 3.3k measured 3.9 k and 22k measure 24k. The final clone sounded just like the original. Oh yea, I also matched the diodes in the ring (these were 1n456 in the three units I had to examine).

I will be getting my hands on another one in about a week, and I will be able to measure the input and output impedances of the transformer. I will also try to get an fft analysis of the filter section.

I can take some voltage measurements off of the unit if anyone needs them, but I doubt the owner will let me unsolder transistors and things like that.

[R.G.]

When I went back to check against the schematic it was clear that the only error was Q1s supply. This really didn't make any difference in the tone of the unit or the function of the switches. I just wanted people to know that the schematic on the internet matched a brassmaster that I had in my possesion.

Good bit of verification. I didn't think the voltage change would make much difference.

I modified my board with the 8 v supply on Q1, changed all the caps in the filter section to ceramic disc (as per original),

Ceramic is highly variable for most non-NPO forumlations.

and changed the resistor values to the actual values of the drifted parts in the original and it was right there. Here are those resistor values - 47k measured 54k (use 56k), 3.3k measured 3.9 k and 22k measure 24k. The final clone sounded just like the original. Oh yea, I also matched the diodes in the ring (these were 1n456 in the three units I had to examine).

Good data; thanks. I think this just affects the exact frequency and peak of the resonant filters, not the overall operations, but good to know.

I will be getting my hands on another one in about a week, and I will be able to measure the input and output impedances of the transformer. I will also try to get an fft analysis of the filter section.

I can take some voltage measurements off of the unit if anyone needs them, but I doubt the owner will let me unsolder transistors and things like that.

Actually, some additional good pictures and especially voltage measurements would make me happy. Ohm's law, some good photos and a little navel-staring make many circuits much clearer. Yes, please, voltages if it's possible for you to get them. DC to ground, all transistor pins.

On the transformer, input and output voltages with a sine wave drive should be plenty. The nominal impedances are not of all that much concern for this circuit.

[DavidRavenMoon]

Hi everyone. I'm the guy that did the copy of the original pcb layout. When I couldn't get a great sound out of my clone, a friend lent me his original. I was convinced that the schematic was wrong at the time, and that the problem was in the filter section. After the schematic and layout were input into the Traxmaker, the computer kept giving me an error between the two. When I went back to check against the schematic it was clear that the only error was Q1s supply. This really didn't make any difference in the tone of the unit or the function of the switches. I just wanted people to know that the schematic on the internet matched a brassmaster that I had in my possesion. It is entirely possible that there are other units revisions with circuit variations.

I modified my board with the 8 v supply on Q1, changed all the caps in the filter section to ceramic disc (as per original), and changed the resistor values to the actual values of the drifted parts in the original and it was right there. Here are those resistor values - 47k measured 54k (use 56k), 3.3k measured 3.9 k and 22k measure 24k. The final clone sounded just like the original. Oh yea, I also matched the diodes in the ring (these were 1n456 in the three units I had to examine).

I will be getting my hands on another one in about a week, and I will be able to measure the input and output impedances of the transformer. I will also try to get an fft analysis of the filter section.

I can take some voltage measurements off of the unit if anyone needs them, but I doubt the owner will let me unsolder transistors and things like that.

That's an interesting idea about revisions.  I had a Brassmaster when they first came on the market, shortly after I heard that Chris Squire was using one... in the music store they even had one on display with a sign that said "Chris Squire's fuzz".  Mine had the schematic glued to the bottom cover, and it's the same schematic that you find everywhere else.  I even have a hand drawn copy I made back then.

But who knows if they changed stuff and left the drawing the same.

At some point I'll try changing parts and see what I get.

[George Giblet]

> just wanted people to know that the schematic on the internet matched a brassmaster that I had in my possesion.

Thanks a lot for your efforts and for sharing the info!

Getting some voltage measurements would be great, but you need to take note of the switch settings (since the bias points depend on the switches.)

I still think it would be a good idea to measure the transformer DC resistance as this would give some idea of the turns ratio.

On your clone, what type of transistor did you end-up using for the filter-amp transistor Q5?  Darlington or the same as the others?

[Gus]

Interesting thread.  Using this schematic that was posted before in this thread
http://files.muziq.be/schematics/maestro_bb-1_001.gif

Besides the filter and bias shift.

  Q2 C to B resistor and the transistor hfe.  What is the Collector operation point?  Q2 sets Q3 operating points and Q3 is the phase splitter diode transformer driver.  Q2 C to B resistor can make a difference depending were the collector "sits" and the resulting wave shapes and headroom.

Q6 operating points set Q7s again Q6 C to B resistor and Q6 hfe might make a difference

Q2,Q6,Q5 and maybe Q4 maybe need to be selected for hfe for good operating points because of the bias setup.   As posted previously in the thread, DC voltages at all the transistor pins would be a help.

Maybe the transformer is not 1:1 etc like others have posted.  If someone had a real BM one could find the ratio and the core size and winding resistance to make a closer clone.

Maybe the filter design is what the designer wanted?

[toneman]

has someone got a mouser number for the little xfmr??

i think i used a 42TM018 for my Lovetone RingModulator clone...
wonder if they would work here also(??)

it's weird how the centertap of the primary is fed back to the bridge(?)
this way, one diode appears across one of the primary halfs(?)

other octave-ups use the secondary for the recitification with only 2 diodes.

as for the "Twin T"....screwyist TT i've ever seen!!
Maybe **that's** the MOJO!!   LOL!!!   
For a "true" (voltage controlled) TT, check out the ancient PAIA 2720-3b.

now it's said that "everything is correct"... ??
think i'll wait for the DVD   LOL!!!

[gigimarga]

I've built these days a ZVEX Jonny Octane clone (which has the same "modulator driver" as Brassmaster) using an 42TM018 (1:1) transformer and it sounds very good for me.