Pioneer A-400 Modifications

[Naughty Nigel]

As mentioned in another thread, I recently acquired a Pioneer A-400 amplifier which I am interested in modifying.

The amplifier already sounds very good in my opinion, but I would like to make it even better, although I am not willing to sacrifice reliability or functionality.

I gather that Tom Evans, the original designer of the A-400 used to offer an upgrade for this amplifier at great cost, but no longer offers this service. Does anyone know what these modification entailed?

My plan at present is as follows:


Replace main smoothing capacitors with better, audio quality types.

Possibly replace some coupling capacitors with polypropylene types.

Adjust output transistor bias so that the amp runs in Class A to about 5 watts per channel.

Possibly replace the Op-amp(s) in the Phono section, although I don't' use the Phono input much at present.

Replace the manky, corroded RCA sockets with gold plated types. (I have cleaned the RCA connectors with T-Cut, but they still don't provide a good enough connection in my view.)

I am also considering replacing the mains transformer with a toroidal type, although I will have to make sure there is enough space for it.



When I have done this I plan to make similar modifications to the A-300, which appears to be virtually identical to the A-400 save for the absence of a MC Phono stage and the less powerful output stage; although the output stage (and mains transformer) would probably be easy enough to upgrade.

If anyone has any knowledge or experience of such modifications I would be very pleased to hear.

Nigel.

[istari_knight]

I had an A400 last year which I modded quite a lot... Of the mods you've listed nos.2 & 4 are the only one's that will make a large audible improvement IME.

TE used to remove the feedback capacitors at the PA input stage & replace them with a DC servo. Pioneer later did this at the factory [A400X] but nobody liked it... Go figure :mental: Personally I prefer the capacitor arrangement as it provides a modder with a great way to tune the amplifiers sound to your taste, I found it to have a larger impact on the sound than the coupling capacitors !

As you probably know apart from the phono equalizer its basically a passive attenuator in front of a PA stage much like a Cyrus 1/2. I recommend the following if you want to avoid throwing money at parts that wont make any difference.

Phono section:

Replace IC101 with either OPA2134 or LM4562.
Bypass C113/C114. [coupling]
Replace C105/C106 with polystyrene. [feedback]
Replace C117/C118 with 1000uF. [psu]

Amplifier section:

Replace C201/C202 with polypropylene. [coupling]
Replace C111/C112 with either very high quality electrolytic like a Silmic II or my choice, a wet tantalum ;) 100uF is fine here but more the merrier. [feedback]
Replace C203/C204 with polystyrene [feedback bypass]

Replacing all of the ceramics with film's is the next best thing to tackle & makes a huuuge improvement bit it is a lot of work.

Of course upgrading the reservoir caps & sticking in a toroidal cant hurt but don't expect a night & day difference [not worth the outlay IMO.] I tried messing around the with bias setting but as much as I wanted to couldn't discern any audible improvement so kept it at factory level.

[Naughty Nigel]

Thanks James, that is very helpful.

The options that you have suggested make good sense, and are easily achievable, so I will try those first. The amplifier must be about twenty years old, so the original electrolytics are probably due for replacement now anyway.

I have the A-300 for comparison, so I will have a listen at each stage to see how they sound. The A-400 has slightly better bass extension than the A-300, but otherwise they are difficult to tell apart.

I will try to update this thread as I go along.

Nigel.

[istari_knight]

No Worries :thumbsup:

If you need anything just shout.

[Mr Kipling]

Tom Evans didn't actually design the A400. It was Doug Randall Pioneer's UK Marketing Manager that finally persuaded Pioneer to produce a stripped down no frills amp for the British market. TE did come up with the GTE version which led to the A400 X. He then worked with Pioneer to produce the A300 Precision.

Whatever happened servos?

[istari_knight]

The A300 / A400 were for the UK market as Stephen says... Elsewhere they are the A337 / A447. Pioneer literally ripped out the tone controls and said "UK special blah blah etc" They are otherwise identical inside ;)

In stock form they're quite average & really need to be partnered with recommended equipment of the time for best results IME [CD63 & Mission speakers] but with a little modding they are quite different animals.

[Mr Kipling]

Stick a couple of VU meters in it. That's what it REALLY wants!

[Naughty Nigel]

Stick a couple of VU meters in it. That's what it REALLY wants!

Oh yes. With bright blue LED's all around them! :eek:

Does anyone know what was different about the A300 Precision? Was it just audiophile grade components in the signal path, or were there more fundamental differences?

I have a service manual for the A-300 and A-400, but cannot find one for the Precision.

Nigel.

[istari_knight]

The A300R was a completely new design - There were varying levels of Precision... Entry level which I owned but when opened I couldn't detect any mods at all... There were a couple of "audio grade" capacitors in select positions but that was it. Then there was the top level [indicated by a blue LED] where the phono stage is removed, big toroid & reservoir caps are fitted & bias is ramped up.

I think I have the SM [I]somewhere, I'll have to have a look on my external drive.

[Naughty Nigel]

I think I have the SM somewhere, I'll have to have a look on my external drive.

Thanks James. That would be brilliant if you have it.

Nigel.

[rtrt]

My apologies for resurrecting your old thread Nigel.

I'd be interested to hear if you went ahead with any mods, how the upgrade process went and of course any resulting changes in the sound.

I'm particularly interested if you had any success with the phono amp section.