Pioneer A400 & Red Hill Audio

omnibeard

New member
Dec 7, 2010
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Hello and happy new year to all!

I've just pulled my old A400 out of the attic (someone else's attic actually, but that's another story), and it's in a bit of a mess. Next to no audio coming out of the right channel, and the front volume knob is a bit skew-whiff. I've always liked the A400, I run a Marantz KI signature CD63 MKII and a Technics 1210 through it.

I can probably open it up and clean it to get the right channel audio back (?) and/or spend some money on having the A400 serviced and upgraded. Red Hill Audio look to do something like this for about £175 plus presumably some post and packaging, so that's going to be the best part of £200 - £250 I would imagine (need to chase them for actual costs).

Does anyone have any experience of Red Hill Audio - I'm guessing the upgrade is going to be something akin to the Tom Evans mod? Or would anyone recommend someone else to do the work?

Also, after spending that amount of money, would I be left with a significantly better amp than either buying something new, or searching for something second hand (which I'm more than happy to do - love a bargain!) and bearing in mind that I do like the sound of the A400 (in the past I've used a variety of Mission and B&W speakers with it and loved the sound of those).

For what it's worth, and call me a heathen, I'm currently running all this through a pair of Cerwin Vega VS10s (this was primarily a party system) but I'm looking to buy something a little more civilised - most likely the obvious Mission 753s if I can find a nice pair.

Thanks!
 
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Anonymous

Guest
Hi, funny we both seem to have a new years resolution to restore the A400.

Firstly I have never used Redhill Audio, so I can't comment. You can buy an A400 for around £150 on ebay in good nick.

It is also worth checking the volume control as it is a ganged knob allowing balance adjustment.

I am about to replace the two decoupling capacitors in position c201 and c202, and have managed to find some Black gate 10uF 50V replacements.

I wanted to ask if people had tried a higher value like this as the original was 2.2uf and impossible to in a sensible size. It looks to me like this cap is DC blocking for the amplifier, but high offsets at the speakers will be protected by the relay protection will they not?
 
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Anonymous

Guest
Hi Guys. The A400 is a pretty good amp when modded, although it lacks dynamic punch because it lacks an active preamp stage.

I'd definitely use film caps for c201/2 instead of those electrolytics. Don't get me wrong, I love Black Gates, but I prefer film-caps in the signal path every time. Don't worry about the size of the caps, 2uF or more is fine in this position and I use 4.7uF Wima which drop straight in.

Cheers, Lee.
 
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Anonymous

Guest
Thanks Lee, but I have already bought some 10uF blackgates for c201 & c202.

I have also increased the Output transistor bias to 85 mA through the 0.66 ohm emitter followers.

Further increase is not possible as one channel will not go beyond 130 ma at which point the amp gets to hot and has no change to the sound.

I am hoping the decoupling caps that will have deteriorated with age will be improved with the new capacitors.

I do have to offer of buying an Arcam Alpha 9 but not sure how much of an improvement would be achieved?
 
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Anonymous

Guest
Black Gates are among the best available electrolytic type capacitors but they are overpriced.

For coupling purposes, I'd prefer MKP type as well. There're lots of fine brands like Wima, Solen, Vishay, Epcos, etc.
 

Mooly

New member
Jun 10, 2011
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Just a word of caution,

Increasing the quiescent current in the (any) output stage is not recommended. For a class ab output stage there is really only one optimal value. You also run a real risk of thermal runaway under hard drive conditions resulting in failure of the output stage even though the amp does have a temperature compensated Vbe multiplier. The heatsinking just isn't suitable for higher quiescent dissipation.

Crossover distortion is a complex issue... increasing the current and thinking it makes it "more like Class A" doesn't work. The optimal value depends on the output stage configuration and the value of the emitter resistors, which are 0.33 ohm for the Pioneer. An optimal value would be around 140ma (around 47mv across each 0.33 ohm) but its asking for long term reliability problems to do it.

The A400 is a great amp but ultimately built down to a price on such as items as heatsinking etc.
 
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Anonymous

Guest
Mooley I have to agree with you. The Pioneer spec says 20mV between the two emitters of the output devices, so 20mV through 0.66 ohm is a mere 30mA and crossover distortion was never noticeable at that level.

I have increased this level to around 60mV therefore increasing bias to approximately 90mA. To be honest I can't really notice a difference apart from an increase in heatsink temperature of approximately 4ºC. This brings quiescent temperature to around 38ºC in a room that's around 23ºC. I don't think thermal runaway will be a problem at this level.

I would also mention the heatsinking is quite a size, also helping in thermal stability.
 
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Anonymous

Guest
I have fitted the Blackgates tonight and have been having a listen to various music to see if I can hear a difference.

I feel the sound seems "Fuller bodied" with maybe slightly better LF registers and slightly less of a cool sound than before(less prominent upper midrange) and just a bit nicer to listen too.

One thing I am sure has changed it the position of the volume control from before. The control is turned down lower than before for the same level, suggesting the previous capacitors were of high ESR.

I measured the Blackgates today at work with a bridge, injecting them with 100hz and they gave an ESR of around 2.5 ohms.

I will try the original caps next week to see the comparison, but a basic reading shows they are still close to spec after 20 years of regular use.

I think there is an improvement, but I want one so brain might be telling me there is.

Anyone who does strip this amp, check the central earthing point that connects to the chassis (black wire) and is connected to the PCB nearby to c201 and 202. Mine had a dry joint and just pulled out!
 

Mooly

New member
Jun 10, 2011
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Heat is the big killer of caps. That and running them close to their maximum ripple current rating. I can well believe the originals are fine tbh

I know from hard won experience that "experimenter expectation" can play a huge role in things like this. If you believe it will sound better then it generally does...

It might be worth trying something non intuitive such as adding a small series (non inductive) output resistor in series with the speaker feed to raise the output impedance. Although that goes against normal logic the subjective results can be good. Maybe something like 0.22 to 0.47 ohms to try.

Edit... the lack of a thread notifier is a huge problem on this forum.
 
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Anonymous

Guest
Yes I have rather hijacked the thread from Omniboard, who I did respond to but not sure if he read any comments thereafter.
 

omnibeard

New member
Dec 7, 2010
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Have been following - but it's all a bit over my head!

My A400 is going to have to wait a while until it gets fixed - I think it deserves to be fixed at some point - as I've just bought a new amp anyway.

Keep it up though!
 
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Anonymous

Guest
Not sure on the series resistor thing, seems the amp is capable of difficult reactive loads, the Epos's are a reasnably easy load.
 

Mooly

New member
Jun 10, 2011
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A series resistor allows the speaker itself to "modify" slightly the actual voltage it sees depending on programme content. A solid state amp is considered to be a "constant voltage" source meaning that the output is unaffected by load. 8 ohm, 6 ohm, 4 ohm and the voltage at the speaker terminal should be the same. Add the resistor and, although the terminal voltage still remains the same the speaker voltage now varies as the impedance varies. Yes, it is distortion of a sort but it can be pleasing to the ear.

Valve amps have a high "series output resistance" due to the winding resistance of the output transformer, and adding a resistor to a solid state amp can go some way to achieving that same "distortion". And some of the best sounding solid state amps include a 0.1 or 0.22 ohm as standard fitted internally.

Reactive loads... to provoke a reactive load you need to bring about rapid changes and reversal of current and nothing in audio (or more correctly music) really does that. Purpose designed test signals might but not music. An exception to that are speakers like the old Quad electrostatics where the impedance falls with frequency to around 2 ohms and the load becomes almost purely capacitive.

However, some loads can be difficult due to impedance dips (my B&W 703's are unbelievably nominally 8 ohm but dip to 3 ohm around 100Hz, and one published figure even quotes a 2.8 minimum).
 
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Anonymous

Guest
I may try the resistor in series but it goes against everything I understand.

I measured the ESR of the original capacitors today, one was 22 ohm and the other 25.

I tried them at 100, 1k and 10KHz, with similar results.

This would explain the increase in amp gain. I have been listening to the amplifier in the last few days and I am convinced on quite a significant improvement.
 
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Anonymous

Guest
[Unpublished by AE after consultation with AC following complaint from Red Hill Audio, pending investigation]

Hi,

I have read your comment with great interest. If I were you I would not entrust your Hi-Fi equipment with Red Hill Audio. I have or at least had an Audiolab 8000p power amp which I sent to this 'man' for 'repair and upgrade' some six months ago. I have had an awful experience, constantly being told excuse after excuse resulting in him never actually doing any work to the amp. Trying to get hold of him on his mobile is a battle in itself as he chooses to not answer his phone the majority of the time and appears unable to return voice messages left. When i have spoke to him he manages to only provide even more excuses as to why he has not completed the work which I had already paid for. It was at the five month stage that I felt it necessary to threaten legal action that he did finally call me back only to explain that although the amp had been working it had now stopped working again following another 'bench test'.

This individual did promise to send the amp back via a courier to a guy at the AIG Audiolab Servicing Group who I had also found via the internet. All was forgiven on my part and so I waited to check with AIG that they had recieved it only to find that it had not been recieved, this I must add was over a week later. Once again I am trying to get in touch with Red Hill Audio but to no avail thus far, numerous messages have been sent via both voice message and text but true to form he does not respond to offer the curtousy of fullfilling this final matter for me.

So in brief I would not trust this man to upgrade or repair anymore of my Hi-Fi equipment, sure his website looks great and when you do manage to speak with him he 'talks a good fight' engineering wise but does not deliver on his promises.

Should I ever get the amp sent to the AIG Service group I can only hope that he hasn't caused the amp irreversable damage so preventing the 'real' engineer from carrying out any further work to the amp as I have since learned can be the case.

I am sorry I cannot give a better review of this man's services but I am simply relaying my past and current experience to you.

Good Luck.

Thanks
 

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