Hi, new poster here
I'm pretty new to the electronic world, but I'm really wanting to get into circuitry and lately have been reading as much as I can to try and learn.
After about a year of reading, I want to go ahead and take the plunge. I figure there's no better way to learn than by tearing something apart and then re-building it (and hopefully making it better in the process).
I have an old Shure m67 4 channel microphone preamp. It sounds awesome, super old-school dirty sound.
What I'd like to do is re-build it into something a little different than it is.
1. I'd like to put each preamp circuit on its own little board, sorta modular style. I realize its probably not necessary, but I feel like I would learn more by doing this instead of simply re-housing a complete circuit.
2. Each preamp has a transformer at the input. If I decided to re-build only 2 channels instead of all 4, could the extra transformers be put to use elsewhere in the circuit? Add some extra saturation? Can it become an output transformer? (really showing how little I know here, huh?)
3. Power Supply. I've read a few different posts that talk about how poor the power supply is for this unit. So, I'd like to rebuild it. The schematic for the preamp uses only 13V for most the circuit, until the output stage, where it uses the full 34V. Someone suggested supplying full 34V for the entire circuit, that this would help with gain/SNR/etc. Does that sound right?
I have other questions about this project but I'll stop there and see if anyone here is interested in helping. I really appreciate any thoughts or advice towards this kinda thing. I'm really hoping this little project will help me learn how audio circuits operate and exactly what each component does to the overall picture. I'm really lost here, but I'm a really good learner and just want to get to the point where I understand this all a bit more.
Thanks for reading this far, cheers!
I'm pretty new to the electronic world, but I'm really wanting to get into circuitry and lately have been reading as much as I can to try and learn.
After about a year of reading, I want to go ahead and take the plunge. I figure there's no better way to learn than by tearing something apart and then re-building it (and hopefully making it better in the process).
I have an old Shure m67 4 channel microphone preamp. It sounds awesome, super old-school dirty sound.
What I'd like to do is re-build it into something a little different than it is.
1. I'd like to put each preamp circuit on its own little board, sorta modular style. I realize its probably not necessary, but I feel like I would learn more by doing this instead of simply re-housing a complete circuit.
2. Each preamp has a transformer at the input. If I decided to re-build only 2 channels instead of all 4, could the extra transformers be put to use elsewhere in the circuit? Add some extra saturation? Can it become an output transformer? (really showing how little I know here, huh?)
3. Power Supply. I've read a few different posts that talk about how poor the power supply is for this unit. So, I'd like to rebuild it. The schematic for the preamp uses only 13V for most the circuit, until the output stage, where it uses the full 34V. Someone suggested supplying full 34V for the entire circuit, that this would help with gain/SNR/etc. Does that sound right?
I have other questions about this project but I'll stop there and see if anyone here is interested in helping. I really appreciate any thoughts or advice towards this kinda thing. I'm really hoping this little project will help me learn how audio circuits operate and exactly what each component does to the overall picture. I'm really lost here, but I'm a really good learner and just want to get to the point where I understand this all a bit more.
Thanks for reading this far, cheers!
Welcome. Assume we can't remember all details of all gear. Give pictures or links to unjam our poor memories. A good overview of the M67 is:
https://pubs.shure.com/view/guide/M67/en-US.pdf
Yes. PLEASE do not cut it up. You can buy a "better" (cleaner, more knobs) mixer for $99 at Banjo World. They will not make any more M67s, and too many have already been cut-up or just plain trashed.
What's wrong with Shure's board? These are TEENY "preamps". Your "modules" could be postage-stamps. Modularizing means more connectors, always a sore-spot in electronics. This is just busy-work.
The Shure Level Loc uses a "mic transformer" backward to get back from line to mike level.
However on the whole, a good (even semi-good) mike transformer is worth more as a mike tranny than any other use. You want to abuse some iron, get $3 modem transformers.
??? The output stage has gain of 500. If it works OK on 30V, then first-guess the preceding stages could eat 30V/500 or much less than 1V. Even allowing 30dB in hand, 2V would cover the signal. Actually those elegant and simple preamps won't bias-up well until 11V-12V, and supply over 15V does not improve performance.
BTW: the BATT terminals are not decoration. We often ran these things with three 9V batts. It is sometimes *very* useful to have a mixer where there is no power. "Souping-up" the amplifiers, higher V and I, just means the battery goes flat before the end of the night. (Actually the M67 is so thrifty it will run for days, and I have done that, on a gig where I could not always be there to check batt.)
I appreciate the input.
At the very least I'd like to add individual outs per preamp. Preferably balanced. Any thoughts on that?
At the very least I'd like to add individual outs per preamp. Preferably balanced. Any thoughts on that?
I guess what I'm really after is some documentation or videos that clearly explain what audio amplification is all about. Laymen terms and such.
Like,
Here's your mic input, hot and cold and grnd
It goes to this capacitor for this reason
It then goes to this resistor for this reason
It then goes to this diode for this reason
...and so on.
I'd like to get comfortable enough with it all to be able to tackle a project similar to my original post, and know what I'm doing without asking. I thought maybe just diving into an already working circuit may help me understand it.
I know theres plenty of kits out there. But that's like a paint-by-numbers. I'll get a finished product out of it, but I won't really learn the why's and how's.
Like,
Here's your mic input, hot and cold and grnd
It goes to this capacitor for this reason
It then goes to this resistor for this reason
It then goes to this diode for this reason
...and so on.
I'd like to get comfortable enough with it all to be able to tackle a project similar to my original post, and know what I'm doing without asking. I thought maybe just diving into an already working circuit may help me understand it.
I know theres plenty of kits out there. But that's like a paint-by-numbers. I'll get a finished product out of it, but I won't really learn the why's and how's.
There are a lot of less respected equipment to cut up that you can experiment with. Those $3 5332 "preamps" on e-bay are a good start. You can change the gain, go for less noise, devise portable and AC power supplies. The box & RF exclusion are up to you.
I learned a lot on a Herald RA-88a mixer I bought for $15 at a flea market. I see those as the Olson RA-88a on e-bay sometimes. Circuit was sort of okay, the packaging was intensely stupid. Hummed like a bee hive. That and the hiss on the RIAA circuit were corrected in my unit for about $20 in parts and a year and a half of experiments. Other old Peavey and Ampeg mixers are as simple, and especially the mono versions don't cost very much.
A good text to start with was "electronic devices the electron flow version" by Thomas L. FLoyd. Mine was $2, a charity shop reject from the local community college because the 2nd edition problems were out of date. Other community college texts are probably as good. I had kirchoff's laws, the RLC math etc in 2nd year physics, but a lot of devices have been made practical and economical since I was in school. Like the op amp, a $150 item when a quarter pounder was $.35. Now you can buy two in an IC for $.38.
I learned a lot on a Herald RA-88a mixer I bought for $15 at a flea market. I see those as the Olson RA-88a on e-bay sometimes. Circuit was sort of okay, the packaging was intensely stupid. Hummed like a bee hive. That and the hiss on the RIAA circuit were corrected in my unit for about $20 in parts and a year and a half of experiments. Other old Peavey and Ampeg mixers are as simple, and especially the mono versions don't cost very much.
A good text to start with was "electronic devices the electron flow version" by Thomas L. FLoyd. Mine was $2, a charity shop reject from the local community college because the 2nd edition problems were out of date. Other community college texts are probably as good. I had kirchoff's laws, the RLC math etc in 2nd year physics, but a lot of devices have been made practical and economical since I was in school. Like the op amp, a $150 item when a quarter pounder was $.35. Now you can buy two in an IC for $.38.
I appreciate your thoughts. Y'all have convinced me to leave the m67 as is... for now 
I still really wanna learn this stuff, and this forum has been the most encouraging so far.
So my friend has an old Sunn Magna 2408. Its already broken. It doesn't power on. Its already got a modular feel, with the way the channels are loaded into the chassis.
So the first thing I need to do; diagnose/repair/replace power supply.
I'll have the unit within a couple of weeks, I'll take it apart, get some pictures, and share them here along with some schematics and such. And hopefully with some help I can start learning how to work on this kinda stuff.
Thanks for the responses so far.
If this is the wrong forum (or wrong site!), point me to somewhere you think would be better suited for helping a noob get his hands dirty.
I still really wanna learn this stuff, and this forum has been the most encouraging so far.
So my friend has an old Sunn Magna 2408. Its already broken. It doesn't power on. Its already got a modular feel, with the way the channels are loaded into the chassis.
So the first thing I need to do; diagnose/repair/replace power supply.
I'll have the unit within a couple of weeks, I'll take it apart, get some pictures, and share them here along with some schematics and such. And hopefully with some help I can start learning how to work on this kinda stuff.
Thanks for the responses so far.
If this is the wrong forum (or wrong site!), point me to somewhere you think would be better suited for helping a noob get his hands dirty.
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Sunn is a good low middle mixer to experment. Not junk, not exactly a classic, large leaded components, .
The difference between a mixer & a preamp, IMHO, is that the channels are on at the same time on a mixer, whereas preamps switch between inputs. Turning inputs on and off can cause clicks & pops, so sliding gain pots up and down is the modern way to do it. The low noise available since the 5532 op amp IC was invented allows this move forward.
A low noise switching technique for line level (2 v 1000 ohms impedance) is the LDR (light dependent resistor), a component that "switches" in hundreds of milliseconds, not microseconds. Clicks & pops have high frequency components, that the LDS's leave out with their slow switchng speed.
This is the place to talk about analog inputs. Happy expermenting.
The difference between a mixer & a preamp, IMHO, is that the channels are on at the same time on a mixer, whereas preamps switch between inputs. Turning inputs on and off can cause clicks & pops, so sliding gain pots up and down is the modern way to do it. The low noise available since the 5532 op amp IC was invented allows this move forward.
A low noise switching technique for line level (2 v 1000 ohms impedance) is the LDR (light dependent resistor), a component that "switches" in hundreds of milliseconds, not microseconds. Clicks & pops have high frequency components, that the LDS's leave out with their slow switchng speed.
This is the place to talk about analog inputs. Happy expermenting.
Adding power to something so sensitive and with so much gain could be a nightmare.
My first USB mixer was laid out very quickly but when I built it up I had 1 VAC of noise on the output with inputs shorted !
The power supply charging impulses were getting into the audio ground and modulating it. I reworked the pcb keeping the power supply separate and had a star ground at the pcb input terminal and all was good.
Cool, at least I won't be offending anyone by learning on it then 😉
What do you mean by "large leaded components"? Are the leads literally bigger than usual? Easier to work with?
I'm not really sure what you're getting at here. I definitely understand the difference between the uses of a lone mic pre vs. a mixer. I simply want to get this unit up and running, learning all I can in the process.
I'll be using the mic pres in a discrete fashion, only using the mix buss for the occasional creative effect. This unit has pre outs on every channel already, so thats a plus.
As far as switching between inputs, I suppose you mean mic and hi-z inputs. I'll never use the hi-z inputs on this unit, I have cleaner units to use for those purposes. So that also opens the possibility of re-purposing that jack per channel to serve some other use. Or possibly just disconnect it and its extraneous circuitry? Would that help "clean up" the audio path for the mic input?
I'm glad I found the right place. Hopefully you all can help guide me thru this learning process, I will be so grateful for even small bits of knowledge and advice.
If a mod feels inclined, it may be a good idea to change the thread title to something more appropriate for where its going. "Sunn 2408 repair" maybe?
If I guess right, Sunn exited the market about 1990, when hand built units with leaded components were common. components that are longer than 1 cm went the way of the dodo bird about 2000. The PV8 mixer witha dead channel I'm trying to fix is full of IC's resistors & capacitors smaller than oats. May not be doable at my age. I'm stocking up on leaded parts before they quite disappear- like when I want one leaded resistor I order 100 of that value. Same with DIP8 IC's: we're all supposed to use $100000 wave solder machines on components without legs. Newer wonder IC's never went through the DIP8 stage.
You keep saying pre-amp, and those are products that switch between the record player, the cd player, the radio, the computer output, the tape player, with a rotary switch or a pushbutton one of seven(or 5 or whatever) selector switch. Devices that change the gain on the various inputs with a volume pot are called mixers. If you're looking for junk devices on e-bay to repair, you'll find a lot more $40 dead mixers than you'll find preamps. Preamps tend to cost $$$$ for the old & rich, like dynaco mcintosh & scott products. Mixers beat up by road bands are full of worn out pots, broken connectors (& overage electrolytic caps, like every other electronic device).
I'd say start a new thread when your Sunn mixer comes in, you can't change thread title IMHO.
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So when I say preamp, I'm speaking from a home-recording perspective. I'm not using turntables/cd players/etc etc. I'm pushing a microphone thru a preamp, and that signal goes to the interface for recording in the PC. No live switching of inputs or outputs. No other gear besides microphones will ever be plugged into the preamp.
That's why I thought this may be the wrong site for this sort of post. I've tried gearslutz for different problems in the past, and those guys over there are... less than helpful. I always get the same responses. Always asking me why-this and telling me shouldn't-do-this, instead of just answering my questions head on. Even an 'I don't know' is more helpful than 'you shouldn't be doing that'
That's why I thought this may be the wrong site for this sort of post. I've tried gearslutz for different problems in the past, and those guys over there are... less than helpful. I always get the same responses. Always asking me why-this and telling me shouldn't-do-this, instead of just answering my questions head on. Even an 'I don't know' is more helpful than 'you shouldn't be doing that'
I checked ebay today for preamps. In all categories, there was one preamp for parts or repair, $534. I'd never heard of the brand.
Under mixers for parts or repair under musical instruments category, there were 30, including a possibly attractive Sunn for $20 + $20 freight, an altec for $28 + $12 freight, and a crown 150 W mixer amp for $20 +$20 freight.
Accurate naming has its rewards.
We hope to be helpful here, especially to green amateurs that might know less than I do. I am no professional repairman, but I've fixed a few electronic devices.
Under mixers for parts or repair under musical instruments category, there were 30, including a possibly attractive Sunn for $20 + $20 freight, an altec for $28 + $12 freight, and a crown 150 W mixer amp for $20 +$20 freight.
Accurate naming has its rewards.
We hope to be helpful here, especially to green amateurs that might know less than I do. I am no professional repairman, but I've fixed a few electronic devices.
I've already got the Sunn 2408 mixer locked down. A friend has it, its broken at the moment, he's gonna just let me have it if I can fix it. It's got 8 preamps and a stereo mix bus. I go visit him this weekend to pick it up.
I can't tell if you're trying to help me buy another unit? Or maybe just sharing relevant information that's slightly on topic? Either way, I'm well aware of what I want/need, and aware of how to search out those units with the proper terms. I appreciate your participation here, just not sure where you're trying to go.
I can't tell if you're trying to help me buy another unit? Or maybe just sharing relevant information that's slightly on topic? Either way, I'm well aware of what I want/need, and aware of how to search out those units with the proper terms. I appreciate your participation here, just not sure where you're trying to go.
Well, single channel, people are buying "USB microphones" that derive their power from the 5 v on the USB port. No expermenting available unfortunately, the boards are under 1 square inch. My fidelity requirements for acoustic piano are so high I haven't experimented with them. I'm using two KSM27 condensor mikes that require phantom power (48 vdc) hence the mixer is required to provide it.
A SM58 or equivalent dynamic mike should run direct into the microphone interface of a PC running Audacity. With a 1/4 phone jack to 1/8 phone stereo plug cable. And for experimenting, those $3 5332 IC preamp board kits on e-bay are a cheap way to start. See this thread about a poster that customized his after studying the circuit: Ebay NE5532 pre amp tone control gain reduction
Getting +-8?dc in and the signal in without hum or RF interference is enough challenge before you start worrying about circuit details.
https://www.diyaudio.com/forums/ana...5532-pre-amp-tone-control-gain-reduction.html
A SM58 or equivalent dynamic mike should run direct into the microphone interface of a PC running Audacity. With a 1/4 phone jack to 1/8 phone stereo plug cable. And for experimenting, those $3 5332 IC preamp board kits on e-bay are a cheap way to start. See this thread about a poster that customized his after studying the circuit: Ebay NE5532 pre amp tone control gain reduction
Getting +-8?dc in and the signal in without hum or RF interference is enough challenge before you start worrying about circuit details.
https://www.diyaudio.com/forums/ana...5532-pre-amp-tone-control-gain-reduction.html
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Alright, we seem to be terribly disconnected concerning the purpose of this thread.
I've been recording for over a decade now. I'm fairly knowledgeable on everything related to audio recording. Gear, technique, etc.
I'm here because I wanna try my hand at some electronic work, and I need some advice. I need advice on circuits, components, etc. I'm not really here looking for thoughts on mics, interfaces, choice of DAW.
Your last post almost reads like stream-of-consciousness centered around recording gear.
Here's my itinerary;
1. Obtain Sunn 2408 mixer
2. Open it up
3. Find out what's wrong with it
4. Fix it
If you feel like you can help with number 3 or 4, that would be amazing. As I said in an earlier post, as soon as I get the thing in my hands, I'll have pictures and initial meter readings on anything that does work, so maybe you guys can help me from there.
I hope I don't sound snarky, just want to keep this thread straight-forward and focused
I've been recording for over a decade now. I'm fairly knowledgeable on everything related to audio recording. Gear, technique, etc.
I'm here because I wanna try my hand at some electronic work, and I need some advice. I need advice on circuits, components, etc. I'm not really here looking for thoughts on mics, interfaces, choice of DAW.
Your last post almost reads like stream-of-consciousness centered around recording gear.
Here's my itinerary;
1. Obtain Sunn 2408 mixer
2. Open it up
3. Find out what's wrong with it
4. Fix it
If you feel like you can help with number 3 or 4, that would be amazing. As I said in an earlier post, as soon as I get the thing in my hands, I'll have pictures and initial meter readings on anything that does work, so maybe you guys can help me from there.
I hope I don't sound snarky, just want to keep this thread straight-forward and focused
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