smurfer77
Super Member
Hi everyone,
I've just imported a Pioneer A-400 over to the USA from the UK. I mainly bought it out of curiosity and wanted to compare to a NAD 3020. After listening for a couple of weeks my impressions are that this is a fun amp. I don't want to get into a big debate about how good it was or wasn't, but I like the amp enough to decide to refurbish/recap. While no amp is perfect, if there was one thing that i would like to improve on the amp it is the bass control. While there is ample bass energy, it is poorly defined. For example, bass stringed instruments that sound like a pluck on my Sansui AU-777A or Dynaco ST-70 (hooked up these two and the A-400 to my amp comparator switch box...) completely lacks the 'pluck', although the overall bass tone/weight is plentiful in volume. Haven't tried on different speakers yet. Anyway, if that factor can't be improved that's fine (I understand improving the bass control might require significant modifications) but any suggestions welcome.
I started searching around for forum threads etc and seen a bit on here, diyaudio and some uk forums, but so far the info I've found is not extensive and much (not all) of it is low in quality and thick with subjective snake oil type reasoning or descriptions. So if someone could point towards some quality threads on A-400 rebuilds that would be great. In any case, I would like to pull together objective info I find (or from AK members) here.
The action points common on a lot of those threads (that actually made some sense) are:
1) replace the two 10,000 uF (50 V) PS caps, C505/6, in case they are approaching end of life;
2) replace the coupling caps at the first stage of the main amp (2.2 uF (50 V), C201/2, typically with film caps. This is targeted because it's just about the only cap in signal path (although if you count phono you could add a few more). Other caps have been replaced by some, but what bothered me was when looking at schematic it seemed almost random which caps were chosen and such forum posts often pushed exotic caps without justification and used subjective terms to describe improvements. So any info on which other caps are really important could be good to pool together here.
3) increase idle current of outputs. Some posts were saying original idle is 20-30 mA and while that wouldn't be unusual the manual asks for 20 mV across the bias resistors (R269/270) of 0.33 Ohm which means about 60 mA was prescribed in service manual. People seem to increase this to as much as 200 mA, but with many opting for about 150 mA being worried about dissipation capability.
There was a popular after market mod (A-400 GTE) done by Tom Evans. It seemed he did similar to the above plus(although some threads indicate coupling caps were bypassed instead of replaced...):
4) lowering the negative feedback by 6db
5) adding a DC servo module underneath the PCB. You can see a pic of that here.
Does anyone know specifically what the modification was made in the feedback loop? Otherwise I will do it my way and report results here. Partly I'm interested in the affect of this mod on bass response. Manual is available on hifiengine
Also any other suggested parts replacements, or known trouble parts, to keep an eye out for? Pics of my unit to come.
I've just imported a Pioneer A-400 over to the USA from the UK. I mainly bought it out of curiosity and wanted to compare to a NAD 3020. After listening for a couple of weeks my impressions are that this is a fun amp. I don't want to get into a big debate about how good it was or wasn't, but I like the amp enough to decide to refurbish/recap. While no amp is perfect, if there was one thing that i would like to improve on the amp it is the bass control. While there is ample bass energy, it is poorly defined. For example, bass stringed instruments that sound like a pluck on my Sansui AU-777A or Dynaco ST-70 (hooked up these two and the A-400 to my amp comparator switch box...) completely lacks the 'pluck', although the overall bass tone/weight is plentiful in volume. Haven't tried on different speakers yet. Anyway, if that factor can't be improved that's fine (I understand improving the bass control might require significant modifications) but any suggestions welcome.
I started searching around for forum threads etc and seen a bit on here, diyaudio and some uk forums, but so far the info I've found is not extensive and much (not all) of it is low in quality and thick with subjective snake oil type reasoning or descriptions. So if someone could point towards some quality threads on A-400 rebuilds that would be great. In any case, I would like to pull together objective info I find (or from AK members) here.
The action points common on a lot of those threads (that actually made some sense) are:
1) replace the two 10,000 uF (50 V) PS caps, C505/6, in case they are approaching end of life;
2) replace the coupling caps at the first stage of the main amp (2.2 uF (50 V), C201/2, typically with film caps. This is targeted because it's just about the only cap in signal path (although if you count phono you could add a few more). Other caps have been replaced by some, but what bothered me was when looking at schematic it seemed almost random which caps were chosen and such forum posts often pushed exotic caps without justification and used subjective terms to describe improvements. So any info on which other caps are really important could be good to pool together here.
3) increase idle current of outputs. Some posts were saying original idle is 20-30 mA and while that wouldn't be unusual the manual asks for 20 mV across the bias resistors (R269/270) of 0.33 Ohm which means about 60 mA was prescribed in service manual. People seem to increase this to as much as 200 mA, but with many opting for about 150 mA being worried about dissipation capability.
There was a popular after market mod (A-400 GTE) done by Tom Evans. It seemed he did similar to the above plus(although some threads indicate coupling caps were bypassed instead of replaced...):
4) lowering the negative feedback by 6db
5) adding a DC servo module underneath the PCB. You can see a pic of that here.
Does anyone know specifically what the modification was made in the feedback loop? Otherwise I will do it my way and report results here. Partly I'm interested in the affect of this mod on bass response. Manual is available on hifiengine
Also any other suggested parts replacements, or known trouble parts, to keep an eye out for? Pics of my unit to come.
