changing transformer on an A400

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I am new to the forum and while I am mostly satisfied with the performance of my Pioneer A400 driving my KEF transmission lines one area I would like to improve is the bass where the amp lacks a bit of grip or drive. My idea is to replace the laminated transformer with a torroidal type as in my limited experience amps with torroidals seem to control the bass better (damping?). Does this idea have legs or am I on the wrong track? Which other components have a larger bearing on the bass control? If I do replace the transformer for a torroidal will any other components need to be replaced with it and should I replace it with one of similar spec/size or go bigger? Thanks for your replies.
Due to the harmonic content of music the mid and treble performance can be greatly influenced by the bass performance.

If you want to have a play then, choose a toroidal that is rated at the same or higher VA then the existing EI. Try also fitting higher capacitance PSU reservoir caps as well, but not at the expense of relocating them as the increased wire lengths will undo any good.

Beware that the amp topology itself can have a lot of influence on the sound and it may simply be that is the sonic signature that you are stuck with.
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Pioneers do sound a bit thin on the bass. Changing the transformer is the wrong direction entirely. Try out a Luxman, Marantz or a few other well made older sets. Then there are amplifiers you can construct yourself. The info is on this board.

Thanks Hugh. Should the diodes be replaced by similar spec ones only more modern designs such as the Schottky, or different spec? Sorry but I know very little about amp design/workings (but handy with a soldering iron). Also any recommendations for brand/size of capacitor?

The good diodes are indeed Schottkys and/or Ultrafast soft recovery, and good caps are just about universal these days, I use United Chemi-con with good results. Some, such as the Nichicon Muse (KG) series, are specially designed for audio, but they are hard to find in Oz. What size are they? I might be able to help you.....

The best diodes, I should add, are the very latest Silicon Carbides from CREE and others, which have zero recovery time. tns plays havoc with noise, which obscures detail in the amp. The disadvantage of the very fast diodes is fragility; exceed the PIV or current rating and they actually explode....


Hugh is right (of course..)!

I've spend a bit of time upgrading an A400. There are many potential areas to improve, starting with fresh power caps, improved dc coupling caps, a squirt of contact enhancer (eg. Caig ProGold) on the input selector mechanism, replacing the rectifier bridge with and hand-built bridge using soft recovery or schottky diodes, etc, etc.

The difference should be night and day. Most of the original character of the A400 should remain (clarity, some say brightness), but quality of bass and perceived noisefloor should be dramatically different.

BTW, the transformer doesn't seem likely to be a bottleneck. It's a nice, big one, so much so that the steel chassis gives a good shudder when powering on!
Hugh, the caps are indeed 10,000uf each - ELNA 50V ACH1152 CEW 85deg. What would you recommend?

mb, thanks for your reply. I would be interested in more detail of your mods. What did you replace the DC coupling caps with? Would you mind sharing the design of the new rectifier bridge? Which reservoir caps did you chose? Can you describe the improvement in sound you achieved after the mods?
For the dc coupling I think I used Elna non-polars + BG NX Hi-Q bypass. The rectifier "bridge" was a small pcb + 3 T220 schottky dual diodes (10A each leg) wired as a bridge.

Additional cap swaps, etc, were using whatever was available in the bin, often Panasonic FCs.

After the mods, sound was even clearer than before, but thankfully the rectifier replacement improved the quality of the bass -- deeper, cleaner. I think that w/o the rectifier swap, the other mods would have made the A400 too bright.

If you're adventurous, you can trace the power supply and make an educated guess where adding some high quality 100uF/50V very low ESR caps can be effective as V+/V- bypass to critical parts of the amp (eg. to the o/p transistors).
Thanks Mervin!! Appreciate your good advice, particularly as you've modded one and likely know far more about it than I do!


I'd recommend Nichicon KGs and UFSR diodes, which I actually have in stock. Mervin's suggestions go quite a bit further, but there is a little more expense involved with the BGs which are exceptionally good though hard to get in Australia.


mb: that is very good information esp re the rectifiers as I am a bit of a bass fiend. I will have to pour over the amp's circuit diagram to identify all the components as I am a bit of a novice around amps. Thank you.

Hugh: I had a look at your website and what a wealth of information you have there! Esp the info on correct sizing of transformers and the sound of different reserve caps. You say that the Spragues have "fantastic" bass but ordinary mids/highs - if one used them with bypass caps which had better highs/mids could you get the best of both worlds? How do the KGs differ from the KMHs and what would they and the diodes cost?
Spragues have "fantastic" bass but ordinary mids/highs - if one used them with bypass caps which had better highs/mids could you get the best of both worlds? How do the KGs differ from the KMHs and what would they and the diodes cost?

Hi Metako,

It really is very difficult to get the best of both worlds; generally you are better off to use better caps, though bypassing does help. I would say Spragues bypassed by BG would be very good, but haven't tried it. Either way this is much more expensive than using a superior cap like the Nichicon KG; bypassing this cap would get better results again, I'd say. The other problem with cheaper caps is noise; it's better to use a KG, follow it with small series resistance, say 100 milliohms, then use a 2,200uF BG. This way the energy reservoir is the Sprague, while the BG is the noise determinant and speaker return current to rail. An even better option is to use a small inductor, typically 4-5mH, between these two caps. The sky is the limit, but then other areas of the amp start to show their weaknesses. PM me if you want prices!


For better bass response also you can look at the following.

If the amp is a feedback type with unity gain at DC (the Jap ones almost always are) then the cap (usually an electrolytic) in series with the gain setting resistor can be replaced with a shorting link.

The thing to watch here is the DC output offset. A lot of these better Japanese amps tend to have dual discrete transistors (in one package) for the input pair resulting in low offset.

The input will likely also have a coupling cap. Removing this will also help the bass.

If troublesome offsets cannot be avoided either from the connecting equipment or by the amp design then replace the mentioned caps with better quality and usually bigger capacity non-polar electros or fix the offsets and avoid the caps (recommended).
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