MXR M87 Bass Compressor Demo

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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Duckman
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Post by Duckman »

http://www.jimdunlop.com/blog/?p=3800&u ... dium=email

I just saw this. Intresting features for an analog compressor in such a small package. SMD rules here, obviously.

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matt239
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Post by matt239 »

MXR Bass Compressor. - Does anybody know what's actually inside one of these?
THAT or DBX chips, or similar? Could be opto, but seems like too many controls..
Sounds pretty good. Sounds good on guitar, but there's not that many demos..

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Post by triplesevener »

I was under the impression that it was FET-based, almost like an 1176 in pedal form. As far as bass compressors go, this one is by far and away my favorite (although I don't own one).

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Post by pedalgrinder »

This one would be a great candidate to get guts shots of it. But i bet it's all smd. Absolute ballbreaker to trace if thats the case. But a schematic of it would definately be nice.
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Post by matt239 »

Bump! :) Anybody seen inside one yet?

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Post by friedtransistor »

Bump again! I really would like to know if it's feasible to make this one or if it's all digital crap...
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Post by pedalgrinder »

I think they are so expensive that no one is willing to even put a screwdriver to it. they are worth $300 there abouts but if you can score the schematic and have the time and patients like $50 bucks but hard to say if anything digital in there.
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Post by kusi »

i have one in the studio, i try to open it and make some gutshots tonight.

cheers!

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Post by kusi »

here some gutshots:


it's a analog 1176-ish fet compressor:
n-ch jfet for gain-reduction (q16 mmbfj113 n-channel) and a discrete gainstage of 3 npn transistors (q14 q15 q17 mmbt5089), similar to a 1176.
didn't check the circuit around the gr-amplifier or the around meter.
looks like there is a switching-powersupply for higher voltage.
input-op is a tl061, output a 2262c.
(sorry no time for more details or even trace it completly xD)

i like the sound of the compressor, i use it all the time while bass-tracking on the di. it's a bit noisier than a 1176-software-compressor, but the general characteristics/functions are similar.

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Post by collingtech »

ok so i got one unit for repair , will try to get the circuit of it, the unit came in bad shape ( someone try to fix the thing after the owner spill some unknow quantity of beer over the unit), the VU create noise ( high pitch) when the leds roll , disabling the vu under the hood ( switch) the compressor seems to be working fine , already check the board i was unable to fix the goddam thing :D , the vu board apears to be workin as it should( appart from the noise) all transistores ok and the caps good too , this week i will trace the circuit :twisted:

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Post by collingtech »

WIN_20191011_11_30_35_Pro.jpg
WIN_20191011_11_30_35_Pro.jpg (75.67 KiB) Viewed 9914 times
ok , im triyng to trace the thing , i found the smd named GY 8 transistor (?) and cant find the reference in smd books, anyone know this piece?

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Post by karul »

both have GY label, but it's different package

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Post by collingtech »

pcb1.jpg
teste.png
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Post by george giblet »

Maybe some hint on the GY / GY 8. Maybe it's a P-channel JFET?

Check out my M78 trace, the input and output circuits have bidirectional tranzorbs, and also P-channel JFETs connecting to ground. I haven't tried to check over your M87 trace but there's a few circuit patterns I see in the M87 that look like the M78,

viewtopic.php?f=1&t=13121&start=80

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Post by george giblet »

So for example in your schematic the collectors of Q19, Q21, Q22 all connect together and go to a cap. This looks like the gates of the P-channel JFETs in my M78 schematic. The source of each of those goes to ground which matches with the bases of in you schematic going to ground.

So I suspect if you drawn in p-channel JFETs the circuit will make more sense.

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george giblet
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Post by george giblet »

OK I think the issue is GY is really 6Y. Then it all makes sense since they are the same JFETs use on the M78!

https://www.newark.com/on-semiconductor ... dp/67R1327

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Post by collingtech »

yep make sense george, i was sratching my head on this input topology, not making any sense , i was thinking about some sort of common base circuit, :D , silly me ,great link george , i will redraw and resume the work , thanks again

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Post by collingtech »

also, whats going on with this varicap on input? d17 and d18, https://pt.wikipedia.org/wiki/Varicap

thats new to me , its some form of Witchery ? :0) , ok going back to work

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Post by george giblet »

also, whats going on with this varicap on input? d17 and d18, https://pt.wikipedia.org/wiki/Varicap

thats new to me , its some form of Witchery ? :0) , ok going back to work
It's not a varicap, it's a tranzorb,
https://en.wikipedia.org/wiki/Transient ... sion_diode

They are used to protect against ESD (electrostatic discharge). There two type bidirectional which are like two zeners connected back to back in series, and unidirectional which are just like tough zeners.

Most modern products have extra junk for ESD and EMI.

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