Morning all - Consensus is that the phono stage in a Pioneer A400 is a bit rubbish and I’m looking to improve on it either by modding the internal one or adding an external one. Found the schematic here although this one does also cover the A400X: PIONEER A400 A400X Service Manual download, schematics, eeprom, repair info for electronics experts
I have so far changed the caps in the phono stage and fitted an OPA2134 - Nichicon Muse electrolytic coupling caps of the original value, polystyrene feedback caps and 1000uF PSU caps (Panasonic FM I think?)
Does anyone have any recommendations for further improvements - the above have already made a really big difference (with the caveat that the amp is ~30 years old and had never had a recap before so may just have benefited from a simple recap). Would a fancy opamp like a Burson or Muses be worth looking at in such a simple circuit?
Coupling caps are 22uf so not sure where I’d go to upgrade them further.
To follow on from the above, would I be best off for the time and effort just purchasing an external preamp? With the mods done I’m not really sure what level I’d need to look to make improvements over what’s in there now.
I have so far changed the caps in the phono stage and fitted an OPA2134 - Nichicon Muse electrolytic coupling caps of the original value, polystyrene feedback caps and 1000uF PSU caps (Panasonic FM I think?)
Does anyone have any recommendations for further improvements - the above have already made a really big difference (with the caveat that the amp is ~30 years old and had never had a recap before so may just have benefited from a simple recap). Would a fancy opamp like a Burson or Muses be worth looking at in such a simple circuit?
Coupling caps are 22uf so not sure where I’d go to upgrade them further.
To follow on from the above, would I be best off for the time and effort just purchasing an external preamp? With the mods done I’m not really sure what level I’d need to look to make improvements over what’s in there now.
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It may be the consensus that's "a bit rubbish" rather than the circuit itself, which seems like a typical 1980s upper midrange combo MM/MC job. Sporting a JFET input stage, it should have good input noise performance for both. This is about as good as phono inputs in lower midrange amps ever got.
I would be very careful swapping the opamp in a composite circuit like this, a faster part than the original may need slowing down via a small capacitor from output to inverting input in order to thwart oscillation in MM operation (I would try 22 pF, 10 pF, 47 pF, an oscilloscope may come in handy).
It is also important that the MM/MC switch be 100% clean (disassembly may be required, I don't think flooding the switch with anything conductive would be a good idea). I would expect the biggest gains in terms of recapping from C111/112.
RF input filtering seems a bit marginal, R105/106 ideally should be a parallel RL combo (Technics amps used to have them by default). I might try 100-220 µH || 100 ohms or so, and if you want to get fancy you could put the old 10 ohms in series with both just in case (a 10 ohm axial resistor tend to remain one up to ~100 MHz, way past the self-resonance of axial inductors as mentioned).
And speaking of input circuitry, make sure that C103/104 and C105/106 are in fact 39p and 220p respectively, as RF immunity regulations in some areas may have mandated impractically large input capacitance to make up for the otherwise inadequate filtering. 259p plus cable is too much for certain cartridges (notably AT) as-is, and input resistance of 56k instead of the more standard 47k is in no way helping matters. If the R-RL mod means you can drop the 220pFs to 100p, all the better.
MC input impedance is a bit on the low side at 100 ohms (R103/104); you may consider swapping that to a slightly larger value like 330 ohms if you have a cartridge that benefits from it.
I would be very careful swapping the opamp in a composite circuit like this, a faster part than the original may need slowing down via a small capacitor from output to inverting input in order to thwart oscillation in MM operation (I would try 22 pF, 10 pF, 47 pF, an oscilloscope may come in handy).
It is also important that the MM/MC switch be 100% clean (disassembly may be required, I don't think flooding the switch with anything conductive would be a good idea). I would expect the biggest gains in terms of recapping from C111/112.
RF input filtering seems a bit marginal, R105/106 ideally should be a parallel RL combo (Technics amps used to have them by default). I might try 100-220 µH || 100 ohms or so, and if you want to get fancy you could put the old 10 ohms in series with both just in case (a 10 ohm axial resistor tend to remain one up to ~100 MHz, way past the self-resonance of axial inductors as mentioned).
And speaking of input circuitry, make sure that C103/104 and C105/106 are in fact 39p and 220p respectively, as RF immunity regulations in some areas may have mandated impractically large input capacitance to make up for the otherwise inadequate filtering. 259p plus cable is too much for certain cartridges (notably AT) as-is, and input resistance of 56k instead of the more standard 47k is in no way helping matters. If the R-RL mod means you can drop the 220pFs to 100p, all the better.
MC input impedance is a bit on the low side at 100 ohms (R103/104); you may consider swapping that to a slightly larger value like 330 ohms if you have a cartridge that benefits from it.
Thanks so much for that super detailed reply, thats really helpful. I’ll give those a try for my next mods on it certainly.
Unfortunately I don’t have a scope, but itd be so useful to have I think I can justify buying one at this point.
In terms of opamps, the (bipolar) Muses02 I was looking at is very similar in terms of specs to the original (also bipolar) chip. I’m not sure if there would be benefits to be gained in using a JFET chip and adjusting the circuit to suit. C111/112 have been replaced with Silmic IIs as part of the full recap of the amp.
I’ll attempt to give the MM/MC switch a good clean too - primarily interested in MM functionality for the foreseeable I believe.
Unfortunately I don’t have a scope, but itd be so useful to have I think I can justify buying one at this point.
In terms of opamps, the (bipolar) Muses02 I was looking at is very similar in terms of specs to the original (also bipolar) chip. I’m not sure if there would be benefits to be gained in using a JFET chip and adjusting the circuit to suit. C111/112 have been replaced with Silmic IIs as part of the full recap of the amp.
I’ll attempt to give the MM/MC switch a good clean too - primarily interested in MM functionality for the foreseeable I believe.
I would change C117, C118 to 470uF.
(With R143 = 56ohm, 220uF, it is LPF at 13Hz, that is too high, must be a few Hz.)
C107, C108, should be 220uF, Low Leakage type.
That will change sound more than changing Riaa Eq caps and opamp.
Suggestions are just copied from bigger Pioneer models, A717 and A616.
And of course, cleaning of that MM/MC switch is very important for good sound, as sgrossklass said.
(With R143 = 56ohm, 220uF, it is LPF at 13Hz, that is too high, must be a few Hz.)
C107, C108, should be 220uF, Low Leakage type.
That will change sound more than changing Riaa Eq caps and opamp.
Suggestions are just copied from bigger Pioneer models, A717 and A616.
And of course, cleaning of that MM/MC switch is very important for good sound, as sgrossklass said.
I'd override the switch contacts with small signal relays doing the actual signal switching but controlled by that oriignal switch.
Note had my head a in 400 recently , but it could be a standard ALPS SPUN series switch which are still available/made today, so just change it.
A quick look at the service manual didn't confirm the full length sadly. , Show me a photo of it
SPUN Series - Push Switch - Alps Alpine
EDIT: Just seen the switch in a a couple of pics and it sure looks like an ALPS SPUN series
https://uk.rs-online.com/web/p/push-button-switches/7582108/
Note had my head a in 400 recently , but it could be a standard ALPS SPUN series switch which are still available/made today, so just change it.
A quick look at the service manual didn't confirm the full length sadly. , Show me a photo of it
SPUN Series - Push Switch - Alps Alpine
EDIT: Just seen the switch in a a couple of pics and it sure looks like an ALPS SPUN series
https://uk.rs-online.com/web/p/push-button-switches/7582108/
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Awesome stuff thank you, going to put an RS order in this weekend I think. Bypassed C113/114 with some Wima MKPs today, maybe some tiny gains, had them lying around though so why not.
Any recommendations on low leakage caps to use for C107/108?
The switch is ALPS and does indeed look like the SPUN type so I’ll also get one of those ordered.
I am somewhat tempted to just remove and bypass the MM/MC switch as I’ve no plans to change to MC for the foreseeable and I can always add a new one back in if I do need to.
Handily, it does look like there are empty pads for inductors next to R105/106 too, marked L105/106
Any recommendations on low leakage caps to use for C107/108?
The switch is ALPS and does indeed look like the SPUN type so I’ll also get one of those ordered.
I am somewhat tempted to just remove and bypass the MM/MC switch as I’ve no plans to change to MC for the foreseeable and I can always add a new one back in if I do need to.
Handily, it does look like there are empty pads for inductors next to R105/106 too, marked L105/106
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I suggest just replace it and then you’ve not altered the design or nobbled it for future resale.
The brand new switch will be good for 5 years plus
The brand new switch will be good for 5 years plus
I’d be inclined to agree with you on the switch - I’d decided to fit a new switch and clean the old one in the meantime until the parts come in - unfortunately destroying it while trying to clean it 🙄
So I just removed the old switch for now, only 2 jumper wires to lock it into MM position so it’ll be easy enough to sort when the new bits arrive.
Cheers for the find on the switch as well, having removed and measured it those are indeed the correct ones
So I just removed the old switch for now, only 2 jumper wires to lock it into MM position so it’ll be easy enough to sort when the new bits arrive.
Cheers for the find on the switch as well, having removed and measured it those are indeed the correct ones
Cracking on with some of this this evening, I’ve managed to find all the stuff in my parts drawer apart from the 220uF low leakage caps - anyone any recommendations on those? I’ll order them along with a replacement ALPS switch once i find some
Think some of the recommendations may be for the A400X, the A400 only has C103/104 at 220p which is still a little high for an AT cartridge so got a 120p to go in after the RL mod
Think some of the recommendations may be for the A400X, the A400 only has C103/104 at 220p which is still a little high for an AT cartridge so got a 120p to go in after the RL mod