Project 3A Build - Discussion
Since I didn't want to hijack sakis's thread about the P3A comparison I decided to open a new thread.
I'm referring to my recently build Rod Elliott P3A stereo amp, described and showed here:
One member asked about the sound, and now the unit had about one week burn in and I discoverd a few things I would like to share / discuss with you.
First of all, the amp is build with very good parts. No fancy high-end stuff, but very solid and reliable "a-grade" insdustry stuff like Vishay/BC capacitors, Wima film caps, thomsson 1% metal film resistors, transistors (including output devices) from ON Semi (Motorola I think), Talema toroid, large Fischer (Conrad) heatsink with 0,7K/W, thick multistranded hookup wire, optimized wire placement and so on.
First I adjusted the quiscent current to 75mA (Rod's suggestion), later to 60mA.
The amps works quite nice. It is absolutely dead quiet. No humm or noise of any kind, no on/off switching noise, nothing!
The sound seems to have quite a lot potential (I will explain the reason for this expression later on). It plays a bit more on the relaxed, musical, powerful and juicy side, it has power, authority and a nice sense for small details, without being analytical. The bass is solid, the midrange fluid and open, the treble is nice and detailed without any hint of grain or harshness. Very nice so far!
But, my source is a Non-Os Philips TDA1543 dac with passive i/v conversion and a passive 20k volume pot, so no active preamp. And this is the first problem. This dac seems to be unable to drive the amp properly. There is always this feeling as if the amp really WANTS to play, but is not able to because of the weak source. This is noticeable because of the weak dynamic abilities, slighly recessed resolution and focus, a rather unprecise and rounded bottom end. I have to add that this dac has absolutely no problems to drive my Anthony Holton nxV200 amps, but perhaps these modules do have a lower input sensitivity and they actually do have a higher input impedance (33k instead of 22k for the P3A).
So, there is the need to build a active preamp :-(
And I was so happy because at the moment my setup is completely opamp-free!
Alright, then there is another problem. The amp produces a lot of heat! Even in idle with the quiscent current set to 60mA, the large heatsink gets really hot after some time. Ok, we do have a very hot period here in germany at the moment, around 37°C for quite some time now, and ok, the heatsink gets very hot, but it stays that way without getting hotter and hotter. But I think it is a bit too much heat, especially in idle mode! I really wonder if this is the normal behaviour of the P3A circuit? As said, it seems to work absolutely fine. Bias is rock solid and stable when once adjusted, dc-offset on both speaker terminals is around 5mV each, the amp is dead quiet and seems to sound great (when a stronger source or a preamp is used), so, do I really have to use a heatsink that is MUCH larger for this small 60-100W power amp?
Here are, for comparison, some pictures of my nxV-200 stereo amp with even smaller heatsinks, and this amp NEVER gets hot, even with 37°C ambient temperature and loud listening levels, and it delivers about the same power as the P3A (slightly smaller transformer but the same rail voltage).
Any thoughts are appreciated!
Thanks a lot!
Heatsink should NOT get very hot at only 60-75 mA current.
Might be that there is some high frequency oscillation going on.
That is the amp is maybe not STABLE.
If looking at schema 60-80W Power Amplifier
there are some important components for stability:
C4 100pF ... you may try increase to 150pF, 220pF to see if this makes anything better
C6 100pF ... probably should be same value as C4
R15 10 Ohm + C7 100nF ... make sure they are attached alright
Of course best is to contact and ask Rod Elliott.
He has probably had other people who have had hot heatsinks in P3A.
-----you have been sugested to change the ceramics you used ....
-----then again none of these projects is really complete unless you look at it with a scope signal generator and some resistive and reactive load
------ many things happen i hoonestly have no intention to insult you but is there a chance that you misscaculated the idle ??? or used a wrong method ???
often if you use an A meter to seee the idle there is a chnace that something wis not proper ...best way is to do both ...check with A meter but also check voltage drop on colector resistors
( what is your rail voltage ??? what is your collector resistors ??? what is the voltage reading across them ??? what outputs ??? what drivers ??? is there any chance that any of them is fake ??? )
i run my P3A one version at 120ma some other at 60 it gets a bit warm but nothing to worie about ...then again my heat sinks are by far smaller ....so something from all is wrong
the suggestion from line up i wouldnt go for.... 220pf in the vas stage will kill many of the nice sonics of the particular amp
post your info and measure and will compaire it with mine and leto you know
kind regards sakis
aditionaly ...some days now i run my P3A with B1 buffer and seems that is a superior combination since it left sound untouched regarding quality and sonics but actually improved the specific area you mention the dynamics
Try replacing the 100nF caps in the supply rails on the amp PCB for 100uF ones. They are for decoupling and seem to be a common cause for the amp oscillating.
One question - why did you build the P3A if you found the NxV-200 so good?
Agreed - Rod should consider redesigning the PCB so that there are 100uF capacitors, preferably close to the output devices.
use of a ceramic capacitor in the driver and vas stage might cause oscilation by far more than a couple of non existing decouplings ...i think ...
If this is the case then you will not be getting proper air flow over the heatsink cooling fins and this will quite easily cause everything to get very hot very quickly. Typicaly if you want to mount a heatsink insode of a case it is a very sensible idea to make sure there are a lot of ventialtion holes both above and below the heatsink fins to alow enough air to flow over the heatsink and to cary away the heat.
No pun intended against Rod, but as a minimum,
an output LR filter is mandatory.
hello wahab why you say that ??? in the P3A C+R filter exists
my friend wahab here i will not agree ....as stated befor i have P3A amplifiers working in an italian dinner wired with simple flat cable tops 2x1mm in lenght some 10 or 15 or 20 or even more driving weird loads and some of them driving piezos ...all this is done in a terrible env. that include dimmers transformers of any kind power and signal cables mixed all together and working perfectly for more than 2years now only with an cR that exists in the circuit and no inductor
the P3A is a perfectly working amp as is and to my understanding the fault lays only in the choise of parts and probably some mistake in tuning ...lets wait of what the OP has to say and we will take it from there ...
other than that is good to talk to you again and hope that you are doing well
wise to think will be that performance versus cost and simplicityis superior
kind regards sakis
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