I bought a little mixer today - Velleman Promix 50S

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I was in a local electronics store buying some parts for my SMPS (finally they got IRF740s in stock today). I had left with my MOSFETs, a bunch of TDA2050s and a new battery for my multimeter, when i remembered that they had this little mixer a while back.

I returned and asked to look at it. 2 line/phono inputs with crossfader, a third separate line input channel and two mike inputs. It also has headphone output. The faders appeared of good quality, 1 year warranty, and they told me none came in for warranty yet. It was $62.50, so what the heck... I bought it. It lacks master level control, EQs or any fancy stuff of that kind, but i figured that it'd be good enough for fooling around at home. Here's a high-res pic of it parked next to my oldschool keyboard. My laptops don't have multiple output sound cards (or at least not without the docks that i do not own) so i had to use my main computer.
An externally hosted image should be here but it was not working when we last tested it.

Full size here.

First impressions - Methinks i'll be voiding that warranty real soon. The sound quality is good, the output level is good, the faders are smooth, the cable of the adapter is of adequate length (btw the adapter has a MONO 3.5mm JACK at the end of the cable, i thought that only i did that kind of stuff...). However the mixer itself is set up really awkward, let me explain.

What was immediately obvious was that the headphone output is taken after the crossfader. How in the world am i supposed to beatmatch with that? I dunno what they were thinking when they designed it. So the first mod would be to patch the headphone amp inputs before the crossfader.

Now for the mike inputs. The mike preamp is of reasonable quality, it has some hiss at max gain but it's able to bring my crappiest chinese dynamic mike to decent levels at just over half gain, so it's good. If anyone is interested i made a recording, i'll link to it here. However there are more annoyances in the mic preamp itself. Let's have a look in the manual.

1. Talk Over Switch (Voice-Over)
DJ MIC reduces the music volume (maximum range: 14dB) by talking. The music volume increases automatically when the DJ stops talking.

This assumes it has a compressor or AGC. Bullsh**! It's a 12dB pad and that's all. You deactivate it by the normal manual way, ie by pushing the switch again.

2. Rotary Balance Control - Pan
The balance control is also a PANPOT-function for the mono signals: its main function consists of mixing the left and right balance in the master output, although it is possible to control each way separately. The PAN can select either of the 2 microphones (L: Mike 1, R: Mike 2)."

I don't think i needed an explanation of what a PAN knob does. And it is not possible to "control each way separately" since the two mike inputs share the same gain knob.

What they mean is that they used a single stereo channel for both mikes instead of two separate mono channels (thus a single gain knob). When the Stereo/Mono switch is in Stereo mode, you have one mic in the left channel and one mic in the right channel. When in Mono mode the PAN knob acts as a "mic mixing" knob. The problem? The Stereo/Mono switch is ON THE MASTER CHANNEL. 🙄 Aka you turn the whole thing to mono if you want one mike in both left and right... Whoever wired this thing up was either drunk or high... or both. :spin:

So the bottom line is... Worth modding? Yes. Worth the price paid? Not quite.
 
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midfi mixer

My Herald RA88a mixer was $15 at the flea market. 7 input, dual RIAA, one stereo high level (CD player) one low level mike. Headphone output works, but I don't use it. You can do better on craigslist, about $50 for a 6 input mixer. Slider quality up to you. Herald function is about the same as yours, but two RIAA inputs for use as a disco hub. I've upgraded op amps and caps for much better sound, but the wiring is even stupider than yours. There are talkover switches on each channel, but only the mike one works. I didn't want to talkover anyway, I wanted a preamp hub to use while I figured out what was wrong with my PAS2 preamp. I'm still using it, the PAS2 runs the battery down on the CD player if the selector is on mag phono (and shorts the other inputs). Now if I can figure a way to make the crossfade slider between turntable 1 and turntable 2 actually pan left to right, so I can make the right channel louder when I am laying on the couch. I'm planning to turn one turntable input to a high level input to I can listen to CD player or FM radio without swapping plugs. I was going to use mike channel for fm radio by changing gain resistors, but the slider is mono and the board is missing traces to the other side of the op amp.
I picked up a Peavey 12 input mixer for $100, but it is going to be a ***** pulling all the knobs off to modify two inputs for RIAA mag phono. Pro mixers are stereo on the output only, individual on the inputs. It is also way huge with lots of pro functions, where as the Herald fits nicely on top of the Steinway next to the turntable and between the two SP2 speakers on stands.
 
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Yea well i don't live in the US so no craigslist for me, and i do have a 12-channel Russian mixer that i bought for next to nothing (like $20), but that's gonna have probably over $200 invested in it until it works properly as it needs all faders cleaned and lubed (not because they make noise, because they have jerky motion), all inputs converted to XLR (and XLRs aren't cheap), new opamps on some channels (had 3 fried channels, it's 2 to go atm), a new paintjob, and other miscellaneous stuff. Besides, it's big and heavy, i wanted something lightweight for fooling around.

Well, at least this one has got new faders. 😛 Anyway, i took it apart today and will post some pics. The mono switch will be easy to rewire (it's connected with jumper wires, i guess they really didn't know what they were doing), it connects together the outputs of the main buffer opamp with two 1k resistors. This only reflects on the output not in the headphones, because they didn't take into account that 1k isn't enough mixing together when you're hitting an opamp input... Solution is simple, replace 1k resistors with 220 ohm or something of that nature, cut the traces to the main buffer and reroute them to the outputs of the mic preamp.

Wiring the headphones before the crossfader is going to require an extra buffer though. I do have a NE5532 on a shelf so i'm gonna use that. By the way, the thing is full of 4558 opamps and cheap electrolytics... Also most of the opamps are in the single-in-line format, will be difficult finding pin-to-pin replacements if i want to upgrade the sonics. But anyway, i have nothing against 4558s so for now i'll let them be.
 
see the following link on my improvements to the RA88a. http://www.diyaudio.com/forums/anal...improving-disco-mixer-mid-fi-performance.html
My mixer came with 4558's, they hissed a bit. I put in phosphor bronze sockets and 33078 DIP op amps at $.68 each, a whole lot quieter. My Peavey amp uses JRC4560's on the input, they also seem to be pretty quiet. (The datasheet has about the same noise spec as 33078, newark was having a special on 33078 with a noise spec in the selector table is the reason I bought them. ) Check both for single inline package. The slider pots on this Herald mixer are perfect- shows the wisdom of "mixing" instead of "preamping" when that is what everybody does. No brand visible. I quit using the PAS2 preamp originally because the volume pot got noisy. I bought a replacement in 1982 from Stereo Cost Cutters, the Dyna bankrupcy high bidder, got an exact replacement that could only be turned with vise-grip pliers on the stem. Rejects stay in the parts room until bankrupcy, I guess. No 250K stereo log taper 1/4 shaft rotary pots available anywhere I can find, and for sure not military quality like the Dyna stuff-28 trouble free years. Installed the stuck pot in the Dyna this spring, when I use it now I use the volume pot on the CS800S amp instead of the one on the preamp.
Craigslist.org operates around the world, although that may not be what people do in your country. Type the URL in, click on worldwide cities, click your capitol, see if anybody is listing. Flea markets are really a growing thing in the US now, with 2/3 of everybody unemployed or underemployed.
 
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Thanks for the reference, will read later. Yes i understand what you mean by craigslist, but here other auction sites are more used. I hadn't actually planned to buy a mixer yesterday, hence i wasn't particularly looking for one. It was an "impulse buy".

Anyway, what i said in my last post isn't totally right. The 1k resistors went to the main buffer, the ones to the headphone amp were higher values, thus they didn't mix in enough. Anyway, modding the mono switch was easy. I chose to wire it to the inputs of the microphone opamp. Now the pan control actually acts as a pan control. 😉 I lost the ability to balance the mikes from one another, but i hardly believe i will ever use two mikes on this thing so that's fine.

For the crossfader thing, i realized that if i would wire the headphone amp to be pre-fader from channels A and B, i would completely lose the ability to monitor the mike and the third channel. Plus, since i did not want to use a summing configuration at the input, i would've had to use a quad opamp. I did not want to use a summing configuration because the faders aren't buffered. They are just simple voltage dividers (with highish values too), and makeup gain is applied afterwards.

Thus i arrived at a compromise. I decided to only monitor the left channels pre-xfader and leave the right alone. So channels A and B together go in the left headphone so i can beatmatch, and if i need to monitor the mike or the other line channel, those go in the right headphone. And i can always use a mono adapter for my headphones if i want to listen to everything together with both ears. 😉

I checked the levels pre-fader and post-makeup gain amp with my oscilloscope, and i figured i would need a gain of 3 for the pre-xfader signals, so that's what i did. I had a lot of 10k resistors so i used a 10k to ground and for a gain of 3 (non-inverting amp so 20k required for feedback resistor) i simply put two in series. I actually used a TL082 as that's what i found first in the parts bin.

Result? Working just fine. 😀 I left the wires so long as i haven't figured out where the little perfboard will go when i put everything back together. 😛
 

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minus grounding

At least all your RCA jack rings are isolated from the case. Are they all grounded together to power supply? - or are they run individually to the minus op amp input. I've been thinking about installing non-case-grounded RCA jacks in my RA88a and separating all the minus sides from case and using the op amp minus input to try to reject some hum with common mode rejection.I still have a little hum. Depends on the day, some days it is dead quiet and other days it rattles a little at 60-120 HZ. The PAS2 is quiet of hum every day, and 12AX7 tubes have nothing like common mode rejection.
 
They are all grounded to the ground of the power supply. The mixer has no hum although the power supply is shoddy at best - 12v AC in (not 12-0-12) and +/-15v out, which falls to +/-13.something when the mixer is getting signal.

Anyway, i figured out where the extra board would fit. I think it ended up rather tidy, quite atypical of me. 😀 I've completed reassembly now and will give it a try in the new config. 🙂
 

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I think I would put a glob of silicon seal or RTV to separate the two PCB's to make sure nothing ever shorts. Other than that, tidy little job. If you have TL082's laying around, you certainly know what to do with a 4558. My datasheets on ST33078 and JRC4560 don't show SIP packages. Just bent lead dip, for SOT wave soldering.
 
Actually, i did insulate the two PCBs, there is some scotch tape but you can't see it because it's transparent. Also the ground of the little PCB is soldered directly to a ground point on the main one.

Anyway, it turns out i had picked the wrong points for the inputs to the new opamp - silly me was testing with a single output soundcard, you can understand the rest. So, i re-wired the inputs to before all faders, straight after the coupling capacitors. This also demanded that i reconfigure the TL082 for unity gain, because now the left was too loud.

Of course, you're gonna say that having no volume controls on the left channel can cause balance issues in the headphones. However, with a signal source with headroom such as a computer soundcard, i can easily balance the levels so that the volume in both headphones is equal just when the meters start going into the red. Thus i can beatmatch AND level match at the same time. 😉 And it's working great so far.
 
I don't worry about the headphone output, because I don't like to sit still that much. I've been searching using a $30 used disco mixer as a replacement for the tube mixer on a Hammond organ. The nightmare would be SIP 4558's like your unit, which I found to hiss objectionably on the RA88a mixer on the quiet parts of classical music or when I had no input. Sometimes I turn the radio off if they do seventeenth century stuff too much. Quiet source is not a problem in disco bars of course, as the mixers were designed for. My mixer now has low noise DIP ST33078 op amps. Much better. Well, farnell/newark doesn't stock any SIP low noise op amps, but mouser.com does in the US. NJM2043 has a noise spec lower than 4560 used in my CS800S amp, though at a different source impedance, so I don't know what that means. $.60 apiece. Mouser also stocks ROHM 4560's in SIP at $.94, but their datasheet link didn't work so I don't know if it is the same noise spec as the TI 4560 DIP only. I've got JRC4560 dip packages as the input of my CS800S amp, and they are quieter than my room ambient if the mixer is off and the amp is on. But anyway, if you ever want to leave your preamp on hooked up to your power amp, but not listen to the hiss of the 4558's, there are some options. I had my disco mixer down to dead silent for a couple of weeks, but it is back to a mild power frequency rattle- time for the isolating of the minus RCA inputs as soon as summer is over, I think. Connecting minus's of phonograph, CD player, FM radio all together to the mixer case makes a big gound loop, I think, and the minus inputs of the op amps are all hooked up to reject common mode noise but not able to due to the case minus bussing. Good luck.
 
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I don't worry about the headphone output, because I don't like to sit still that much.

Well, neither do i, even as a DJ i often get onto the floor and dance (doing random parties only and not having DJ'ing as a nightly job at a club has its advantages 🙂), however when i use that headphone output, i want it to work right.

I've been searching using a $30 used disco mixer as a replacement for the tube mixer on a Hammond organ. The nightmare would be SIP 4558's like your unit, which I found to hiss objectionably on the RA88a mixer on the quiet parts of classical music or when I had no input.

I don't listen to classical music too much, but the area where the 4558 really starts getting annoying is the mic preamp. It hisses a bit more than i'd like. However it's just fine for wiring to my computer when i'm bored and trying to rap. Pro audio outputs -> home audio inputs = win. Little gain required means no hiss. 🙂

Another weak point is the headphone output, which is produced by... you guessed - ANOTHER 4558. No transistors to help it, no nothing. It is able to do 150mW at 8 ohms, but most headphones are 32 ohms you know. Also the Altec Lansing headphones i happen to be using are very clear and detailed, but need copious amounts of bass boost, and at this chapter the mixer performs at the level of a $10 MP3 player. Though, i am afraid that a serious mod to the headphone amp would bring the already frail power supply to its knees, so maybe i'm better off just buying headphones with more bass.

My mixer now has low noise DIP ST33078 op amps. Much better. Well, farnell/newark doesn't stock any SIP low noise op amps, but mouser.com does in the US. NJM2043 has a noise spec lower than 4560 used in my CS800S amp, though at a different source impedance, so I don't know what that means. $.60 apiece. Mouser also stocks ROHM 4560's in SIP at $.94, but their datasheet link didn't work so I don't know if it is the same noise spec as the TI 4560 DIP only.

The only "outside" sources i can use are Farnell and TME. I'll see what i can find on TME. But for now it's fine really - and i'm alerted by some discontinuities in the xfader when it's slowly brought from the extreme right side. I'm no expert in mixers, but, instead of doing passive mixing and applying makeup gain, wouldn't it be better to buffer the signal before and hit the faders with a higher level signal? Of course, this would probably require a major rebuild, possibly an entire new PCB... and i'm not in the mood to do that.
 
PCB's are easy??

I did a PCB once in the seventies, involved a lot of pasting of stickers on a transparent sheet, then some photochemicals, then some evil brown fluid ferric sulphate that we poured down the drain, probably very illegal now. I don't want to do that again. If you see a Herald RA-88a for Euro 20 jump on it, mine has 2 ea NEC 575C2 amplifiers with little heatsinks for the headphone output, which is apparently big enough for my Koss 8 headphones. Just passing some numbers on the quiet op amp IC's, I'm sure somebody over there stocks NJM (apparently new name for JRC) parts. I've got a single channel Ampeg mixer that would do the job on replacing my organ's tube mixer, but in 1962 technology it weighs twenty kilos and is huge, and organ weighs 200 kilos already, so something modern and modifiable is in order. There are a lot of Numark disco mixers coming up on the used market here, and searching diyaudio, apparently they don't issue schematics or parts list, so who knows how hard they are to upgrade to okay hiss.
Oh, the RA88a amps the inputs and then mixes with the sliders, at least on the mag phono that I have traced out, so yours does it passively? Weird.
 
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I did a PCB once in the seventies, involved a lot of pasting of stickers on a transparent sheet, then some photochemicals, then some evil brown fluid ferric sulphate that we poured down the drain, probably very illegal now.

Haha. Nah man, no pasting of stickers now, there's those magical things known as OHP markers. Or if you have a laser printer just print it mirrored on photo paper, apply to PCB with clothes iron, wet the paper and peel it off et voila.

I don't have a laser printer so i do them by hand with the OHP marker - heck, my SMPS controller has a board full of 1206 SMDs, when i look at it's kinda hard to believe i did the PCB AND all soldering by hand. Okay the 1206s were unsoldered from a motherboard by means of a hot air station, but all soldering to my PCB was done with a plain 25W iron, coz using the hot air station with leaded solder = blowing other parts away before you manage to get one attached.

And yes i did use "the evil brown stuff", it's ferric chloride actually. Safe to pour down the drain once you're done with it, if you put lots of water in it beforehand, otherwise it'll corrode your pipes. 😛

If you see a Herald RA-88a for Euro 20 jump on it, mine has 2 ea NEC 575C2 amplifiers with little heatsinks for the headphone output, which is apparently big enough for my Koss 8 headphones. Just passing some numbers on the quiet op amp IC's, I'm sure somebody over there stocks NJM (apparently new name for JRC) parts.

Quite possible, i'll have to look. I don't believe i'll find the exact mixer but might find something close to it.

There are a lot of Numark disco mixers coming up on the used market here, and searching diyaudio, apparently they don't issue schematics or parts list, so who knows how hard they are to upgrade to okay hiss.

I would avoid Numark if one is to use the mixers for their intended purpose. The faders crap out rather quickly.

Oh, the RA88a amps the inputs and then mixes with the sliders, at least on the mag phono that I have traced out, so yours does it passively? Weird.

Yeah, this passive mixing was the reason why patching to before the crossfader didn't work. Basically what the xfader does is pull the other channel to ground, hence why there's no buffering before it as it would either burn the faders or the buffer amp. Speaking of funny things - On Velleman's site it says they'll reply in 3 days, i even got an automated response assuring me that my email has been noted.

It's been like what - 3 weeks? No reply. Maybe they're too ashamed of their failure. 😛
 
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