Rebuilding (re-cap'ing) a Phono Mixer

[Matt Syson]

Hi
There is no point in atempting too much 'high end' modifications since the source and circuitry is only mediocre. Panasonic or whatever caps would be fine and metal oxide resistors. NE5532 or 34 are not particularly bad op amps and probably better than the originals although you should get the lower noise specced versions. There are many others to choose from so fit GOOD sockets and you can play. Ectra RF bypasses would probably help.
Unless you have the equipment to actually fully test your mods limit your enthusiasm to simple replacements. Different op amps may oscillate.
Have fun
Matt S

[Minion]

Actually the JRC 4558DD Opamps are quite sought after for distortion pedals as they were the Ones used in the Original Ibanez "Tube Screamer" pedal which is a Pretty Kick ass pedal if you get the original model with the JRC 4558D opamp.....

Actually my really crappy Traynor guitar amp used a 4558 chip and it had a awefull sound which I was able to Improve considerably by replaceing it with a OPA2134 which might be a Good choice for your Mixer ...If you do go for the NE5532 (Which is a Pretty Good chip if used properly) you might try putting a 47pF ceramic cap between the Supply Pins which will help with stability with circuits not designed arround the NE5532......

Recapping the Electro"s might be a good idea if the Mixer is old , ecpecially if you Bypass them with some 0.1uf Poly"s, i don"t know about redoing the resistors as the Johnson noise from the Resistors is probably below the Noise floor of the of the Circuitry especially if it has a not well filtered and unregulated PSU , actually Increaseing the Value of the Filter caps in the Power supply with low leakage types and bypass them with some good Polly caps will probably have a Larger impact than many other Mods like the resistors......

I am actually contemplateing tearing open my Mixer which is a Really Crappy one (BST Labs36 8ch mixer) and seeing what Mods I can do on it as I have to open it up anyways because I believe a few Solder joints have come apart as I loose Ballanced operation on some Channels when I adjust the Gain knob......


Well good luck with your project.......

Cheers

[demonfuzz]

Thanks for replies...
I fitted the PCB with DIP sockets for the IC's/opAmps last nite & installed new gold RCA jacks, so far so good, just gotta do the caps & it will be complete.

I had some OPA2134 & NE5532/5534's laying around.
I have most the caps already (Xicon brand, they'll do for now) but am debating ordering more/different caps as I don't have all the exact values, which leads me to my question...

You metioned replacing some of the current caps with ".1uF"?

In each section of this mixer (Mic pre, Phono 1&2, Headphone & Stereo out) there are about 4-6 caps (electrolytic/16v).
The values are 22uF, 4.7uF & 100uF.
(the power supply section has 8 x 220uF but I'll leave those values alone).

What would switching the values do & how do I know when swithing the value to a higher or lower uF value would help?

I know when I re-capped my console a tech told me to replace all the caps in my particular console with 100uF's across the board & it came put sounding great.
I don't know if the same would apply here.

If it does, then thats extra cool as I have a ton of Xicon 100uF caps laying around.
But just don't know if I should use exact values to replace or can I use a different value?

Thanks alot again!

[Matt Syson]

Hi
The 0.1uFs would go in parallel with the electrolytics, so replace the electros then probably put the 0.1s on the solder side of the board.
Replacing all caps with 100uF may not be a good plan here as some roll off of very low frequencies is desirable. You may simply end up with your bass drivers flapping in a strong wind with turntable rumble.
Matt S

[Minion]

Yes, You would not replace the elctros with 0.1uF Poly"s you would bypass them with 0.1uF Poly which means put them in Paralell with the Electro"s....

You could either put them on the other side of the Board or you can connect them to the Leads of the Electro"s and keep them on the Same side of the Board ,That is if you have the Space (Axials are sometimes better for this because they have longer leads)...If space is really a concern you could use 0.1uF Metalized Poly caps which are considerably smaller and offer simular performance.....


In the power supply if the Eight 220uF Caps are basicly just 4 caps in series on each of the Positive and Negitive rails then you could replace 2 of the 4 with say 470uF or even 1000uF 50v Caps and then put the 0.1uf Polys were the other 2 220uF caps were which would greatly increase the Filtering and Lower the DC Ripple.....


In most cases you can Replace Most electro"s with Higher Value ones as long as they aren"t in the audio Path and ones used for Power supply Filtering can allmost allways be increased with a expectation of better performance......

One thing you shouldn"t do is use caps that are rated close to the Voltage used, Meaning if it is a 12v or 15v PSU you shouldn"t use 16v Caps as Capacitors will have much more Leakage and will not be nearly as efficient when run near there Max Voltage Limits....You will see in a LOT of cheaper gear that uses Caps that are just barely above the Voltage specs and they do this to save space and Money but it also degrades the performance and the longevity of the device......

In the Power supplies that I build (Usually +/-15v to 18v with +48v Phantom power) I use at least 1000uF 50v caps bypassed with 0.1uF Polystyrene caps on the +/- Power rails and 1000uF 100v caps on the +48v Phantom rail) and these are dead quiet PSU"s....



Cheers

[demonfuzz]

Originally Posted by Minion

In most cases you can Replace Most electro"s with Higher Value ones as long as they aren"t in the audio Path and ones used for Power supply Filtering can allmost allways be increased with a expectation of better performance......

I *really* appreciate all the info fellas...
I will try some larger value caps & see what happens with the PS circuit.

When you say long as they aren't in the audio path, does that mean any part of the actual audio circuit, or only the actual outputs from the PCB (say the Stereo out lines).
Kind of not clear on this.

With each of the Phono stages, there are 6 caps, 22uF, 100uF, 4.7uF (2 each).
In the dinky little summing circuit (stereo out) there 8 caps, 4.7uF (6 of them) & 100uF (2 of these).

Far as the different values, does higher uF value = more freq response & lower uF = less?
(Is there a standard formula / law for calculating this?)

You mentioned the bass rolloff may be affected, which in a phono mixer I understand why you'd want to rolloff (i.e. rumble filter).
This does ring familiar though, as when I re-capped my console the very 1st thing I noticed was the bass was massively improved & opened up, but in that case it was very desireable!

I'll probably try putting the .1uF Poly's in parallel after a test run - I've seen this before in other gear & didn't understand why it was done .

Again thanks, just trying to get a grip on this before I go too far.

[Minion]

Originally Posted by demonfuzz

I *really* appreciate all the info fellas...
I will try some larger value caps & see what happens with the PS circuit.

When you say long as they aren't in the audio path, does that mean any part of the actual audio circuit, or only the actual outputs from the PCB (say the Stereo out lines).
Kind of not clear on this.

With each of the Phono stages, there are 6 caps, 22uF, 100uF, 4.7uF (2 each).
In the dinky little summing circuit (stereo out) there 8 caps, 4.7uF (6 of them) & 100uF (2 of these).

Far as the different values, does higher uF value = more freq response & lower uF = less?
(Is there a standard formula / law for calculating this?)

You mentioned the bass rolloff may be affected, which in a phono mixer I understand why you'd want to rolloff (i.e. rumble filter).
This does ring familiar though, as when I re-capped my console the very 1st thing I noticed was the bass was massively improved & opened up, but in that case it was very desireable!

I'll probably try putting the .1uF Poly's in parallel after a test run - I've seen this before in other gear & didn't understand why it was done .

Again thanks, just trying to get a grip on this before I go too far.

When I say in the Audio Path i mean the actual audio signal going through the Capacitor...You generally don"t want Electro"s in the audio patch because they create a unpleasant distortion at higher Frequencies, If you must use Electro"s in the audio path it is generally recomended that you bypass them with a Polly cap which helps with the High frequency distortion as Poly caps do not have this Problem but are Impractical to use were high uF values are concerned because of there size.....

Also the Value of the Cap if in the audio path is a concern as the Caps can act as a Filter at higher values and filter out frequencies in the audio band.....

I do not actually have the info on hand for what values filter what frequencies but a Lot of EQ Schematics have a Chart that shows when values to use for certain frequencies....

Generally Electro"s in the audio path are usually there for blocking DC from getting into the audio signal so sometimes high values are used because they are needed for Blocking DC but in well though out circuits they solve this by preventing the DC from getting into the audio in the first place which can be hard to do especially with preamps that have Phantom power were +48v DC is injected directly into the audio wires to power Condenser mics so they use high value electro"s to block this DC from getting to the Input on the Opamp....


Cheers