You have mentioned your problem, and you have referenced the situation to a Pioneer A-878. So your problem is not because you plug in the wrong transistor, or have the amplifier underbiased or the like. But your expectation is high, at least as high as the Onkyo amp whose spec (it uses 2SK389-BL dual JFET) is better than the P3A
No, power supply bypass will not fix the problem. Bypassing will only makes the issue more pronounced.
The bright sound is the typical characteristics of BJT amplifier like this. You have to tweak the amp like Sakis did with his P3A, until you find the "balance". (But don't forget about the buffer I mentioned before)
Its just possible that the very large input cap (which is not suitable for this pcb, BTW) might just be picking up RF from the output inductor. This creates significant instability and makes the amp sound very glassy and harsh.
I have experienced this on my own amps during testing...... don't move away from the component sizes selected by the designer.
Hugh
I have experienced this on my own amps during testing...... don't move away from the component sizes selected by the designer.
Hugh
If RF coupling between the input capacitor and the output inductor (or other parts of the circuit) is suspected, the OP could temporarily position the capacitor at the other end of the input cable (10-20 cm or more should work) and short out the C1 PCB position. Increasing capacitor-to-board separation in this way should identify whether this is the cause.
I read the posts, I did not see whether OP has a scope and generator. If so, sweep the audio band frequency from low to high. If there is lack of phase margin, verge of oscillation, you usually see peaking of the frequency. That could make the amp brighter.
feed square wave and see the ringing. Take picture of the scope and post here. It's all guessing so far.
feed square wave and see the ringing. Take picture of the scope and post here. It's all guessing so far.
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No, all set-up requirements including bias current setting, are described under the heading Powering Up. In case you didn't understand, the bias current that equates to a 55mV measurement across Q7-Q8 collectors, is approximately 80 mA.ok, i thinked he says about offset
Quote from paragraph 4:
"Current is approximately 1.5mA / mV, so 50mV will represent 75mA quiescent current."
yes yes understanded, the drop @ the resistors.
and this is very good because is easier to measure its the trick here.
is it possible to measure current with inductor ?
if is possible this means i should make a pic wattmeter with lcd and only two probes (with two inductors @ the probes ).
you measure the voltage with the probes and current wirelessly with the inductors.
P.S. another thing im planned, wattmeter with the voltage from these .22 .33 .47 resistors.
with lcd again and pic microcontroller this can be protection too
and this is very good because is easier to measure
its the trick here.is it possible to measure current with inductor ?
if is possible this means i should make a pic wattmeter with lcd and only two probes (with two inductors @ the probes ).
you measure the voltage with the probes and current wirelessly with the inductors.
P.S. another thing im planned, wattmeter with the voltage from these .22 .33 .47 resistors.
with lcd again and pic microcontroller
this can be protection too
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Thank you all for the advice. Meanwhile, I have changed C1 and the sound is about the same, however I think it is what Jay is saying. I am comparing this to the Onkyo and it may just be a bit too much. I will look into your advice and replace the power supply with something suitable and see what I can mod from there. Thanks !
That is not good to hear. It has been my ambition for a few years to finally finish this amp and have it as my main front amp replacing the Hafler 220 I have presently.
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I have read other opinions about this amp and harshness was not a common feature. I don't think you should be concerned and just get and finish your amp. I'm not familiar with the sound of the Hafler. In comparison, a new amp might sound brighter just because an existing amp is slightly recessed.
a lot has to do with the music content and the amplifier that you used to hear from many years back
If your amplifier was Quad or Audio lab or Musical fidelity yes you might find a P3A very strange to hear from
Had a client 60 years old that used to hear Class B amplifiers and rock music all his life
He had a perfectly made enclosure twin supplies and rugged 4X 22.000uf but the actual amplifier was toasted
We installed 2 modules a classic EF2 circuit with 3 sets of outputs biased about 70 ma each at least 100k bandwidth and very fast ...
---In the first place the client complain for bass .... In reality his previous amplifier had a very poor bandwidth unable to be flat up to 12K and above and obviously now with an amplifier that was 100K bandwidth highs was above bass to his ears ...Returned it to reduce that bandwidth strictly to 20 K and he was happy with that ...
---Few days later and after the "bass" problem was solved the client complain about having no character even though clean and accurate but no character in the sound ...
I got the massage from the first "complain " and returned the amplifier and i reduced the bias from 70ma to 5 ma ...to increase crossover distortion ...
Got a very happy client who has no idea about amplifiers .....but at least he has a fat wallet ....
If your amplifier was Quad or Audio lab or Musical fidelity yes you might find a P3A very strange to hear from
Had a client 60 years old that used to hear Class B amplifiers and rock music all his life
He had a perfectly made enclosure twin supplies and rugged 4X 22.000uf but the actual amplifier was toasted
We installed 2 modules a classic EF2 circuit with 3 sets of outputs biased about 70 ma each at least 100k bandwidth and very fast ...
---In the first place the client complain for bass .... In reality his previous amplifier had a very poor bandwidth unable to be flat up to 12K and above and obviously now with an amplifier that was 100K bandwidth highs was above bass to his ears ...Returned it to reduce that bandwidth strictly to 20 K and he was happy with that ...
---Few days later and after the "bass" problem was solved the client complain about having no character even though clean and accurate but no character in the sound ...
I got the massage from the first "complain " and returned the amplifier and i reduced the bias from 70ma to 5 ma ...to increase crossover distortion ...
Got a very happy client who has no idea about amplifiers .....but at least he has a fat wallet ....
Premature Speculation
As I see it, the state of the P3A construction by Ravenash is:
1. The PCB doesn't look like Rod's current version. So best of luck with that.
2. The picture of the testing jig is so small it's impossible to discern anything except that there are lots of loose jangling wires connecting everything together. Not the best environment for amplifier stability. Hope the power supply and heatsink are adequate. Is the heatsink grounded?
3. We have never heard if Ravenash did any checks to see if the amplifier was oscillating. I wonder why?
It seems to be a bit premature to generalize/speculate about the P3A until it has been established that Ravenash's amplifier is in fact operating normally.
Regards ...
As I see it, the state of the P3A construction by Ravenash is:
1. The PCB doesn't look like Rod's current version. So best of luck with that.
2. The picture of the testing jig is so small it's impossible to discern anything except that there are lots of loose jangling wires connecting everything together. Not the best environment for amplifier stability. Hope the power supply and heatsink are adequate. Is the heatsink grounded?
3. We have never heard if Ravenash did any checks to see if the amplifier was oscillating. I wonder why?
It seems to be a bit premature to generalize/speculate about the P3A until it has been established that Ravenash's amplifier is in fact operating normally.
Regards ...
Finding out whether an amplifier is working properly is fine for those with the gear ($$$$) and experience to measure but many DIYs are just builders and tinkerers who can only hope that their work will emulate the prototype closely enough. A cheap DMM, soldering iron and a few basic hand tools is often all that is available.
The obvious fix is to use the original PCBs and avoid the unknowns until a lot more is learned about PCB layout than shown by some others who have already posted their efforts here. Some seem to have been autorouted or guessed, which might result in a neat and tidy layout but with little consideration to shielding of supply EMI or unwanted coupling between the stages. Also, many begin without realising that in CFP designs, thermal coupling is needed between the Vbe multiplier and drivers rather than the output transistors.
FWIW, I also have built P3a a few times, from both Rod's PCB and my own humble stripboard layout, where I did have problems until sorting out the location of the input stage and feedback route. Then, all was fine with no more wriggly lines, hash or glitches on the output into a resistive load.
As a single aid to troubleshooting, even a 20 MHz analog 'scope can help make a lot of problems so much easier to identify but unless you have an ongoing need and the interest, it's hard to justify the cost and time needed to learn how to get the most from it. A USB or PC Soundcard analyzer can be superb too, given decent analysis software, quiet PC and wideband card to begin with.
As mentioned, it's no less important to start with a good power supply layout too, with tightly twisted leads for the power supplies, neatly routed away from signals and also twisting the input and output pairs with short, well spaced runs. Otherwise, you'll be struggling with such construction basics rather than knowing what the amplifier potential really is. It should be excellent, even with standard parts quality and its simple design, as many here agree
The obvious fix is to use the original PCBs and avoid the unknowns until a lot more is learned about PCB layout than shown by some others who have already posted their efforts here. Some seem to have been autorouted or guessed, which might result in a neat and tidy layout but with little consideration to shielding of supply EMI or unwanted coupling between the stages. Also, many begin without realising that in CFP designs, thermal coupling is needed between the Vbe multiplier and drivers rather than the output transistors.
FWIW, I also have built P3a a few times, from both Rod's PCB and my own humble stripboard layout, where I did have problems until sorting out the location of the input stage and feedback route. Then, all was fine with no more wriggly lines, hash or glitches on the output into a resistive load.
As a single aid to troubleshooting, even a 20 MHz analog 'scope can help make a lot of problems so much easier to identify but unless you have an ongoing need and the interest, it's hard to justify the cost and time needed to learn how to get the most from it. A USB or PC Soundcard analyzer can be superb too, given decent analysis software, quiet PC and wideband card to begin with.
As mentioned, it's no less important to start with a good power supply layout too, with tightly twisted leads for the power supplies, neatly routed away from signals and also twisting the input and output pairs with short, well spaced runs. Otherwise, you'll be struggling with such construction basics rather than knowing what the amplifier potential really is. It should be excellent, even with standard parts quality and its simple design, as many here agree
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