1 deck, 2 preamps? - UK Vintage Radio Repair and Restoration Discussion Forum

PsychMan · 22nd Mar 2016, 10:09 am

This isn't really a vintage question, but Im wondering if anyone could advise all the same. I suppose it could also apply to a more eccentric vinyl setup

Im looking at how I can connect a turntable signal to 2 preamps to send the signal both into a computer and a mixer at the same time. This is for DJ use, and to enable something known as timecoded vinyl, whilst retaining a traditional setup of 2 decks and a mixer. Im more comfortable doing this with a line level signal, but I want to use the phono preamp built into the mixer.

Cartridges in question are MMs (Shure M44-7). I wondered if a simple buffer amp operating in unity gain would be advisable between the signal and one of the preamps so as to not load the signal too much?

Radio Wrangler · 22nd Mar 2016, 10:42 am

Don't split the signal at the cartridge output level. It's too low and you'll have trouble with hum, also the cartridge borks best into a controlled impedance.

Go from the cartridge into one of the phono preamp converter boxes, set for moving magnet (not moving coil 'MC') and it should have enough output to drive both the computer audio input and a flat input on your preamp.

If youmust use the phono preamp in the mixer, you'll need to get an output from the phono RIAA stage output before the mixer does anything else to it.

David

22nd Mar 2016, 12:01 pm

Use the tape out socket of the preamp (if it has one) for the computer.

PsychMan · 22nd Mar 2016, 12:11 pm

Thanks for the input guys, I shall have to have a think.

Radiowrangler's comments have confirmed my fears of splitting such a small signal. I will see if I can add an output connection to the mixers RIAA stage, this would also be the neatest solution and cheap to achieve.

The issue with using external amps is the mixer is 2 channel, but each channel can have a switchable line or phono level source. So I couldn't physically run 4 line level signals into it (I.E regular turntable plus digitised source x 2), but I could send 2 line level signals and 2 phono signals.

IanBland · 22nd Mar 2016, 12:23 pm

You could construct a simple non-inverting unity gain buffer with a dual op-amp if you don't mind constructing one. Then split the signal from that, i.e. just have dual outputs paralleled on your buffer box. Or be really flash, use a quad op-amp, and have separate buffers to each output channel. The current would be low enough to comfortably run off a PP3.

jamesperrett · 22nd Mar 2016, 3:47 pm

What mixer are you using? There may be a way to split the signal after the preamps. Otherwise you could use external RIAA preamps into the line inputs on the mixer and the computer.

PsychMan · 22nd Mar 2016, 3:57 pm

Its a Gemini PS540, around 12 years old or so. I will need to open it up and clean all the controls soon anyway so will take a look.

The one concern with this is when switching to line level, whether the mixer keeps the RIAA element operating at the same time. I would expect it does, but not certain.

The line level comes from the computer, but the computer needs the phono signal with the control record to function and control what it is doing for cueing and pitch adjustment. So while the mixer is only working with the line input in terms of the audio it is mixing, it still needs to perform the preamp function, if that makes sense? Apologies if not, its made my head spin a little thinking about it!!

EDIT: I would need to use the phono preamps in the mixer, because I can only have 1 phono or 1 line (switchable per channel). So the theory is I can use line level when using the computer software, or switch to phono and use the decks and mixer as originally intended.

cmjones01 · 22nd Mar 2016, 4:30 pm

Having looked at the mixer's instruction manual on line, it doesn't seem to have any way of getting the output from the preamp other than through the usual output, which is of course affected by the faders and so on. You'll have to hack into the mixer to do what you want.

The good news is that the mixer will almost certainly keep the RIAA stage operating even when that input is switched to 'line'. You might even find that the preamp output is available at the pins of the line/phono switch, which would make it very easy to find and bring out to the PC.

Chris

PsychMan · 22nd Mar 2016, 4:39 pm

Thanks Chris, yeah the mixer has 2 line level outputs, Master and Record out, and no way externally of getting to the individual signals. Sounds like a great excuse to have a muck about with it!

Hopefully as you suggest the signal will be easy to find, and I can add some opamps and phono sockets to give me a line out for each channel

nigelr2000 · 22nd Mar 2016, 4:56 pm

I modified a KAM mixer recently to make a 4 channel mike mixer, it's similar to the Gemini and the phono input went through an extra op amp with RIAA then to the line/phono switch before going to the mixer stage input so you could lift a signal off the input to the switch as the phono stage stays live in both positions.

dseymo1 · 22nd Mar 2016, 5:35 pm

Does your mixer have FX send/return sockets?
If so, you could use those for the computer feed.

Scimitar · 22nd Mar 2016, 5:50 pm

I use a phono buffer that also has a USB output for the computer. Wasn't very expensive. It connects after the pre-amp and before the power amplifier.

Scimitar · 22nd Mar 2016, 5:54 pm

Behringer UCA202 U-Control Ultra low-latency 2 In/2 Out USB/Audio Interface

Cost me £23.00

IanBland · 22nd Mar 2016, 9:39 pm

I've got one of them. Pretty handy bit of kit for the price.

PsychMan · 23rd Mar 2016, 4:08 pm

No effects loops to use unfortunately.

I had a look inside yesterday, there seems to be ample space to drill the chassis and add some surface mount phono sockets.

I found a black ribbon wire cable going to the switches with 6 wires, Im guessing 2 channels plus 1 ground for each source. shouldnt take too much to identify which channel is which

The PSU may be a little trickier for me to work out, as I'd like to steal a tiny amount of current for 2 dual op amps to operate in unity gain to provide my line outputs. With no labelling or documentation I may have to dig around a bit. Depending what voltages I get from the power supply, I suppose I could either take a dual supply (if present) and use that as is (if within spec). Or just take one positive supply and get a dual supply with a few resistors.

Diabolical Artificer · 24th Mar 2016, 7:24 am

Should think they'll be a dual supply for the balanced/mic ins, that's assuming your mixer uses op amps instead of tfmr's. BTW, could you not use a small tfmr to split the signal? I know one would be out of phase with the other, but can be swopped back with a op amp.

Andy.

PsychMan · 31st Mar 2016, 2:58 pm

Well I identified a line level signal for each deck on 3 pin connectors. L and R with ground in the middle, nice and logical.

But upon testing it, the output works fine when the mixer is set to phono for that channel. When it's switched to line however, the right channel drops out and the sound in the left gets very quiet. Any idea why this might be? At this stage I haven't used any op amps, I just wanted to see how it worked without.

I've presented the outputs on 2 mini stereo jack sockets, and these have come straight off the ribbon cable that runs from the "Input board", to the signal select switch. As far as I can tell I have the correct inputs and they sound great when source is set to phono, sadly not much use for me. Perhaps this mixers design doesn't make it suitable?

In the end I did this to a Gemini 626 pro, as my 540i has another fault. Service data for this is poor and detail impossible to see when zoomed in

PsychMan · 6th Jun 2016, 2:11 pm

Well, I managed to get a decent schematic for the mixer, and I can see why the phono output stops when line level input is selected.

The connector from the "in/out PCB" (containing RIAA stage) is a 6 pin connector. 2 pins for ground, 2 pins for Phono, and 2 pins for Line. The source selection switch appears to be a 4 pole switch and when switched to Line, it sends the output from each phono preamp channel to ground, and connects the line source to the inputs on the master mixer board (and vice versa when switching back)

Im wondering what exactly is the reason for grounding those outputs and why they couldn't simply remain disconnected?? I was thinking I could get round this if I were to add an op amp running in unity gain from the phono preamp, feed that to the main mixer board, and pinch my output before this device?

I've attached 2 images to demonstrate what I mean, 1 of the preamp section showing the connector pinout, and a 2nd part of the main PCB section showing how the switch is wired.

julie_m · 7th Jun 2016, 7:41 am

It's probably done to prevent breakthrough of unwanted signals. You don't want faint traces of CD in the background while playing vinyl, or vice versa.

PsychMan · 7th Jun 2016, 11:03 am

Thanks Julie, I suspected that but was afraid to suggest it incase I was talking rubbish. Its nice to hear it from someone as knowledgeable as yourself.

The switches are quite compact, and perhaps there isn't as much "insulation" as one might like? Or perhaps this is a problem due to a very densely populated PCB. It is after all a "budget" mixer. If I had a spare 800-1000 quid I'd invest in a Pioneer DJM 850

I've taken some of these same switches off a scrap mixer, I was thinking to add these somewhere on my mixer for consistency and to allow me to switch out the source BEFORE the Line / Phono switch and thus circumvent the shorting to ground.

But, if the switches themselves are an issue like this, I may have the same problem Gemini circumvented by shorting them. I'll have to do some experimenting before I start drilling out the case for more switches!

22nd Mar 2016, 10:09 am	#1
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	1 deck, 2 preamps? This isn't really a vintage question, but Im wondering if anyone could advise all the same. I suppose it could also apply to a more eccentric vinyl setup Im looking at how I can connect a turntable signal to 2 preamps to send the signal both into a computer and a mixer at the same time. This is for DJ use, and to enable something known as timecoded vinyl, whilst retaining a traditional setup of 2 decks and a mixer. Im more comfortable doing this with a line level signal, but I want to use the phono preamp built into the mixer. Cartridges in question are MMs (Shure M44-7). I wondered if a simple buffer amp operating in unity gain would be advisable between the signal and one of the preamps so as to not load the signal too much?

22nd Mar 2016, 10:42 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: 1 deck, 2 preamps? Don't split the signal at the cartridge output level. It's too low and you'll have trouble with hum, also the cartridge borks best into a controlled impedance. Go from the cartridge into one of the phono preamp converter boxes, set for moving magnet (not moving coil 'MC') and it should have enough output to drive both the computer audio input and a flat input on your preamp. If youmust use the phono preamp in the mixer, you'll need to get an output from the phono RIAA stage output before the mixer does anything else to it. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

22nd Mar 2016, 12:01 pm	#3
Guest Guest Posts: n/a	Re: 1 deck, 2 preamps? Use the tape out socket of the preamp (if it has one) for the computer.

22nd Mar 2016, 12:11 pm	#4
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: 1 deck, 2 preamps? Thanks for the input guys, I shall have to have a think. Radiowrangler's comments have confirmed my fears of splitting such a small signal. I will see if I can add an output connection to the mixers RIAA stage, this would also be the neatest solution and cheap to achieve. The issue with using external amps is the mixer is 2 channel, but each channel can have a switchable line or phono level source. So I couldn't physically run 4 line level signals into it (I.E regular turntable plus digitised source x 2), but I could send 2 line level signals and 2 phono signals.

22nd Mar 2016, 12:23 pm	#5
IanBland Hexode Join Date: May 2015 Location: Northampton, Northants, UK. Posts: 380	Re: 1 deck, 2 preamps? You could construct a simple non-inverting unity gain buffer with a dual op-amp if you don't mind constructing one. Then split the signal from that, i.e. just have dual outputs paralleled on your buffer box. Or be really flash, use a quad op-amp, and have separate buffers to each output channel. The current would be low enough to comfortably run off a PP3. Last edited by IanBland; 22nd Mar 2016 at 12:33 pm.

22nd Mar 2016, 3:47 pm	#6
jamesperrett Octode Join Date: May 2007 Location: Liss, Hampshire, UK. Posts: 1,873	Re: 1 deck, 2 preamps? What mixer are you using? There may be a way to split the signal after the preamps. Otherwise you could use external RIAA preamps into the line inputs on the mixer and the computer.

22nd Mar 2016, 3:57 pm	#7
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: 1 deck, 2 preamps? Its a Gemini PS540, around 12 years old or so. I will need to open it up and clean all the controls soon anyway so will take a look. The one concern with this is when switching to line level, whether the mixer keeps the RIAA element operating at the same time. I would expect it does, but not certain. The line level comes from the computer, but the computer needs the phono signal with the control record to function and control what it is doing for cueing and pitch adjustment. So while the mixer is only working with the line input in terms of the audio it is mixing, it still needs to perform the preamp function, if that makes sense? Apologies if not, its made my head spin a little thinking about it!! EDIT: I would need to use the phono preamps in the mixer, because I can only have 1 phono or 1 line (switchable per channel). So the theory is I can use line level when using the computer software, or switch to phono and use the decks and mixer as originally intended.

22nd Mar 2016, 4:30 pm	#8
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,677	Re: 1 deck, 2 preamps? Having looked at the mixer's instruction manual on line, it doesn't seem to have any way of getting the output from the preamp other than through the usual output, which is of course affected by the faders and so on. You'll have to hack into the mixer to do what you want. The good news is that the mixer will almost certainly keep the RIAA stage operating even when that input is switched to 'line'. You might even find that the preamp output is available at the pins of the line/phono switch, which would make it very easy to find and bring out to the PC. Chris __________________ What's going on in the workshop? http://martin-jones.com/

22nd Mar 2016, 4:39 pm	#9
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: 1 deck, 2 preamps? Thanks Chris, yeah the mixer has 2 line level outputs, Master and Record out, and no way externally of getting to the individual signals. Sounds like a great excuse to have a muck about with it! Hopefully as you suggest the signal will be easy to find, and I can add some opamps and phono sockets to give me a line out for each channel

22nd Mar 2016, 4:56 pm	#10
nigelr2000 Heptode Join Date: Nov 2008 Location: North Walsham, Norfolk, UK. Posts: 900	Re: 1 deck, 2 preamps? I modified a KAM mixer recently to make a 4 channel mike mixer, it's similar to the Gemini and the phono input went through an extra op amp with RIAA then to the line/phono switch before going to the mixer stage input so you could lift a signal off the input to the switch as the phono stage stays live in both positions.

22nd Mar 2016, 5:35 pm	#11
dseymo1 Retired Dormant Member Join Date: Dec 2007 Location: Shropshire, UK. Posts: 3,051	Re: 1 deck, 2 preamps? Does your mixer have FX send/return sockets? If so, you could use those for the computer feed.

22nd Mar 2016, 5:50 pm	#12
Scimitar Heptode Join Date: Oct 2015 Location: Gloucestershire, UK. Posts: 719	Re: 1 deck, 2 preamps? I use a phono buffer that also has a USB output for the computer. Wasn't very expensive. It connects after the pre-amp and before the power amplifier.

22nd Mar 2016, 5:54 pm	#13
Scimitar Heptode Join Date: Oct 2015 Location: Gloucestershire, UK. Posts: 719	Re: 1 deck, 2 preamps? Behringer UCA202 U-Control Ultra low-latency 2 In/2 Out USB/Audio Interface Cost me £23.00

22nd Mar 2016, 9:39 pm	#14
IanBland Hexode Join Date: May 2015 Location: Northampton, Northants, UK. Posts: 380	Re: 1 deck, 2 preamps? I've got one of them. Pretty handy bit of kit for the price.

23rd Mar 2016, 4:08 pm	#15
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: 1 deck, 2 preamps? No effects loops to use unfortunately. I had a look inside yesterday, there seems to be ample space to drill the chassis and add some surface mount phono sockets. I found a black ribbon wire cable going to the switches with 6 wires, Im guessing 2 channels plus 1 ground for each source. shouldnt take too much to identify which channel is which The PSU may be a little trickier for me to work out, as I'd like to steal a tiny amount of current for 2 dual op amps to operate in unity gain to provide my line outputs. With no labelling or documentation I may have to dig around a bit. Depending what voltages I get from the power supply, I suppose I could either take a dual supply (if present) and use that as is (if within spec). Or just take one positive supply and get a dual supply with a few resistors.

24th Mar 2016, 7:24 am	#16
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: 1 deck, 2 preamps? Should think they'll be a dual supply for the balanced/mic ins, that's assuming your mixer uses op amps instead of tfmr's. BTW, could you not use a small tfmr to split the signal? I know one would be out of phase with the other, but can be swopped back with a op amp. Andy. __________________ Curiosity hasn't killed this cat...so far.

31st Mar 2016, 2:58 pm	#17
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: 1 deck, 2 preamps? Well I identified a line level signal for each deck on 3 pin connectors. L and R with ground in the middle, nice and logical. But upon testing it, the output works fine when the mixer is set to phono for that channel. When it's switched to line however, the right channel drops out and the sound in the left gets very quiet. Any idea why this might be? At this stage I haven't used any op amps, I just wanted to see how it worked without. I've presented the outputs on 2 mini stereo jack sockets, and these have come straight off the ribbon cable that runs from the "Input board", to the signal select switch. As far as I can tell I have the correct inputs and they sound great when source is set to phono, sadly not much use for me. Perhaps this mixers design doesn't make it suitable? In the end I did this to a Gemini 626 pro, as my 540i has another fault. Service data for this is poor and detail impossible to see when zoomed in

6th Jun 2016, 2:11 pm	#18
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: 1 deck, 2 preamps? Well, I managed to get a decent schematic for the mixer, and I can see why the phono output stops when line level input is selected. The connector from the "in/out PCB" (containing RIAA stage) is a 6 pin connector. 2 pins for ground, 2 pins for Phono, and 2 pins for Line. The source selection switch appears to be a 4 pole switch and when switched to Line, it sends the output from each phono preamp channel to ground, and connects the line source to the inputs on the master mixer board (and vice versa when switching back) Im wondering what exactly is the reason for grounding those outputs and why they couldn't simply remain disconnected?? I was thinking I could get round this if I were to add an op amp running in unity gain from the phono preamp, feed that to the main mixer board, and pinch my output before this device? I've attached 2 images to demonstrate what I mean, 1 of the preamp section showing the connector pinout, and a 2nd part of the main PCB section showing how the switch is wired. Attached Thumbnails

7th Jun 2016, 7:41 am	#19
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: 1 deck, 2 preamps? It's probably done to prevent breakthrough of unwanted signals. You don't want faint traces of CD in the background while playing vinyl, or vice versa. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

7th Jun 2016, 11:03 am	#20
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: 1 deck, 2 preamps? Thanks Julie, I suspected that but was afraid to suggest it incase I was talking rubbish. Its nice to hear it from someone as knowledgeable as yourself. The switches are quite compact, and perhaps there isn't as much "insulation" as one might like? Or perhaps this is a problem due to a very densely populated PCB. It is after all a "budget" mixer. If I had a spare 800-1000 quid I'd invest in a Pioneer DJM 850 I've taken some of these same switches off a scrap mixer, I was thinking to add these somewhere on my mixer for consistency and to allow me to switch out the source BEFORE the Line / Phono switch and thus circumvent the shorting to ground. But, if the switches themselves are an issue like this, I may have the same problem Gemini circumvented by shorting them. I'll have to do some experimenting before I start drilling out the case for more switches!