The Brassmaster Mysteries

[mountainking]

Hey guys, I just wanted to let you know that I am now determined to figure this thing out and over the next few days I'm gonna try to do some experimenting and I'll get back to you with my results. Until then, here are some things that I'm thinkin' after doing a little experimenting last night. I will do my best to explain this in a way that will make sense.

First, I think that the brass switch connections on the factory schematic http://filters.muziq.be/files/schematics/maestro_bb-1_001.gif are drawn correctly. If you look at it, you'll see that each pole of the switch puts one of the resistors into the circuit while taking the other one out. On RG's schematic one resistor is always connected in the circuit and the switch simply connects the other resistor in parallel with it. Now someone with a real brassmaster could easily figure this part out for us by looking at the brass switch and letting us know what kind of switch it is and how many of its terminals have something connected to them. Also, the factory schematic shows (parts list in upper left hand corner) that the brass switch is a dpdt switch. This obviously is another mistake, although I'm really starting to think that these mistakes/errors were intentionally done. Doesn't it seem strange that the only part of the circuit that isn't drawn carefully and in an easy to understand way, is the filter section?

Second, I'm pretty sure that there is a cap missing from the schematic. With the changes that I had suggested in the other thread, there needs to be a coupling capacitor between the twin t filter and the "filter amp." Without a coupling cap the "filter amp" will get mis-biased and will result in some bad sounds. I've tested this theory. With the cap, deep solid low end heaviness, without the cap, harsh nastiness and other bad things.

Anyway, I probably won't get a chance to get back online til early next week, so hopefully when you hear back from me I'll have some more solid information for you guys. Take it easy.

Alan

[SonicVI]

Here's a pic I had saved from a long time ago. Unfortunately I have no trace side.  I count 17 caps, which is how many are in the schematic.

[R.G.]

OK, I got out the schemo and the circuit simulator.Yes, there are several errors in the factory schematic. It ... can't... work properly that way.

Here are a couple of big ones:
1. The switch is a 3PDT slide, as the photo shows and as the schematic shows, not a DPDT as stated on the parts notes.
2. I believe the 82K at the base of Q4 is on the wrong side of the 0.1uF cap leading to that base from the transformer.
3. I thought there should be another cap between the 150K attached to switch 2 and the pole of switch 3. It turns out that I'm probably right, but that it makes not too much difference in the waveforms coming out for small signals. That's a maybe. I'm guessing that this is the cap that Alan thought was missing. Maybe so. Where did you put your cap, Alan?

I think that if I did another layout, I'd use either a CD4066 or CD4053 switch to do the three sections of SW3.They're all at a reasonable bias voltage and should work fine. An SPST or SPDT would then select the filter frequency.

The filter amp produces a peak at either 700Hz or 3.2kHz depending on the Brass 1/Brass 2 setting. Harmonics high/low dumps in more bass content when the switch is connected. I'm guessing that's the "harmonics low" setting.

The diode ring and transformer modulator produces odd squarks of ringing on every other zero crossing. I may not have had the signal levels right.

[vortex]

R.G. The diode ring and transformer modulator produces odd squarks of ringing on every other zero crossing.

Squarks?!

I have no idea what a squark is but I'm intrigued!

[George Giblet]

The filter type and twin-T connection needs to be resolved first!

This thread redraws the filter connection errors as a twin-t notch filter:
http://www.diystompboxes.com/smfforum/index.php?PHPSESSID=b42a513333f2785ed4ecd0558b791fb8&topic=56690.0

This thread redraws the filter connection errors as a twin-t band-pass filter:
http://www.diystompboxes.com/smfforum/index.php?topic=48636.0

My understanding is the "brass" voicing is a band-pass filter.   RG's comments imply that too.

[R.G.]

My understanding is the "brass" voicing is a band-pass filter.   RG's comments imply that too.

I'm sorry, I wasn't being clear. 

It is resolved. It is definitely a bandpass filter, the twin T being a notch filter in the feedback path of the filter amp. The notch in the feedback path is converted to a forward loop peak, just as in the Twin T wah at GEO.

The peaks are either about 19db at 3.2kHz or about 8db at 700Hz, depending on the Brass 1/2 setting.

The picture helps as well. I used too big a transformer. I could have picked a smaller one, but I was trying for as much bass response as possible, it being a bass effect.

[lowstar]

hi r.g.,

The filter amp produces a peak at either 700Hz or 3.2kHz depending on the Brass 1/Brass 2 setting. Harmonics high/low dumps in more bass content when the switch is connected. I'm guessing that's the "harmonics low" setting.

did you run the simulation with the circuit as in your schematic/layout ?
does the harmonics hi/lo work in both settings of the brass hi/lo switch in your simulation ?
if the filter works as it is supposed to in your layout (=if the filter part is right in my build), i could try to make those changes that you suggested in points 2 and 3 and listen to the result...

funny how this thread fell asleep again...

cheers,
lowstar

[Meanderthal]

 So... basically, so far it's the factory schematic's  switching, move 1 cap, and add another...? Am I missing something?

[shuriken]

Hey everyone, I'm fairly new around here, but watching this thread with interest. Hoping someone can sort out this mystery once and for all. I made one from GGG, and like everyone else got it partially working. Even partially though, this sounds great on bass, so I'm hoping for a fully functional unit. Thanks to everyone for providing this info!

[$uperpuma]

someone should ask Josh at Malekko what schematic he and Barker based the B:Assmaster on.  I hear its a pretty Convincing clone.

[R.G.]

funny how this thread fell asleep again...

I guess it depends on how urgently you want a brassmaster how funny it is.

I'm waiting to see what mountainking thinks before I do any work that might then need to be trashed.

[SonicVI]

someone should ask Josh at Malekko what schematic he and Barker based the B:Assmaster on.  I hear its a pretty Convincing clone.

They reversed it from an actual pedal. He hasn't told me wha's wrong with the GGG/Geofex layout but he's offered to mod my clone for me so it must be something pretty simple that's off.   My switches both make big changes in the sound so I haven't taken him up on it yet. I may next time to drive over to Austin.

[Meanderthal]

 Does anybody actually have a non- cloned one? The pic SonicVI posted is the first and only time I've ever seen any part of one...(thanks for that, very interesting!). Just curious... because a pic of the other side of the PCB(and a tedious reverse engineering) could clear this up a bit...

Yeah, I guess that's a stupid question, but I went and asked it anyway...

[R.G.]

Does anybody actually have a non- cloned one?

Yep, that's really the question.

I can diddle the factory schem into what I think it should be, but who knows what they really were?

[Meanderthal]

 Then again, if it sounds good, I guess it dosen't make any difference. As I see it, there's a possibility that an educated guess would sound better than  the origional...

Origional isn't always necessarily better... Even Edison didn't always find the BEST way to implement his inventions, although he would never dream of admitting that.

[George Giblet]

>It is resolved. It is definitely a bandpass filter, the twin T being a notch filter in the feedback path of the filter amp.

Thanks for the clarification. 

[mountainking]

Hey guys, sorry it took me so long to get back to you, but I've been real busy this past week. Looks like some you figured out some of the same things I did. I was hoping to get back with all of the answers, but honestly all I really have to offer is more possibilities. I spent several hours experimenting so I guess I might as well tell you guys what I did. Hopefully it won't just be a confusing collage of words.

My initial thought was that the actual twin t filter was passive and that it was connected between the "filter drive" and the "filter amp". The first thing I did was to tack just the filter circuit on to the output of another bass fuzz that was already on my breadboard and it gave it a nice deep sound. Next I breadboarded the brass master(with the filter hooked up the way I described in the first sentence). It sounded lousy, lots of harsh highs and very little low end. So I sat back and stared at the factory schem some more. I noticed(as mentioned) that filter was connected in the feedback loop of the "filter amp" which would make the filter active. I had noticed this before, but the filter section of the schematic is so screwed up that I figured that this was also a mistake. Well I figured wrong, because once I placed the twin t filter in the feedback loop of the "filter amp" (look at the colorsound inductorless wah schem) I got some cool resonant filter sounds and the brass and harmonic switches both seemed to change the sound in a useful way. Thanks to SonicVI we know that the brass switch is definitely a 3pdt, which makes sense, and that there are no caps missing from the schematic. Let me try to explain again why I thought there was a missing cap. Between the output of the "filter drive" and the input of the "filter amp" there needs to be a dc blocking cap to avoid mis-biasing problems. Now since there are no missing caps from the schem then the .05uf must go where I thought the "missing cap" was suppose to be right?, but on the schem it seems to show the .05uf on the output of the "filter amp" which made sense to me, I just thought the "filter amp" should have a input AND a output cap, although in my experiments leaving out the output cap didn't cause any real problems.

I spent alot of time plugin', pullin' and rewiring, listening for something that I realized that I wouldn't recognize if I heard, since after all, I had never played through an original brassmaster, and I eventually got to the point where I thought, "this is gonna take a hell of a lot more time to decode then I thought it would and I really have to get back to working on my own designs", so I ripped the mess off the breadboard and got back to my things.

Determination...frustration....surrender.

I hope something I wrote was useful in some way, sorry if it wasn't. Maybe we'll have to wait for some crazy person to take apart his(or her) rare and valuable brassmaster and tell us its innermost secrets. I'm gonna go now and drink some(more) beer. Have a good weekend, I'll check back in when I get a chance.

P.S.-Hey R.G., I heard those odd squarks.

[mountainking]

Then again, if it sounds good, I guess it dosen't make any difference. As I see it, there's a possibility that an educated guess would sound better than  the origional...

Origional isn't always necessarily better... Even Edison didn't always find the BEST way to implement his inventions, although he would never dream of admitting that.

Very good point by the way.

[sonic_tooth]

Bump.... anyone ever figure this out??

[fogwolf]

Does anyone just have a Paul Barker Assmaster they could compare to the schematic to see what was missing? I understand the change was very minor.