🙂 sigh. It seems I'm being shot down for trying to compromise here and there and trying to go for no setup. Perhaps I should change out one of my diodes for a P600. I should have a few lying around, and it should give me much less quiescent while still maintaining no-setup. Else I'll just swop out those diodes for a biasing circuit. It will actually be easier to just go for the biasing. Ai ai ai.
But the hum! I must still be rid of it.
I'm going to sort that ground path, but perhaps the lower bias will actually get rid of the hum altogether. I measured the hum on my meter, and it only read 0.01 Vrms. Maybe if I can reduce quiescent PSU ripple, I'll get rid of that hum. I mean, I have about 7 different amplifiers lying around that I've built recently, and none hum whatsoever, and the difference is biasing. And by reducing quiescent current to 30 - 60 mA, I'll reduce total ripple to about 100 mVp-p, rather than 1.3 Vp-p (previously I quoted an incorrect figure for this, but I've re-calculated). Results to follow.
I reduced quiescent current and disconnected the PCB ground connection. Hum is very soft now, but still unacceptable. I believe it comes from wiring, and I'll redo it sometime, but I don't have time now.
The good news is that I've disconnected the preamp, and the output is silent, but for the slight hiss. The bad news is that I've disconnected the preamp. Now i just need my preamp to work!
Now things are looking up! No hum, and the preamp is working. The system is working.
What i did is do it as designed - separate transformer for the preamp.
What i did is do it as designed - separate transformer for the preamp.
So you are saying that all the hum came from the Zenered preamp supply? Hmmmmm......
No. I'm still using that supply, just as designed with another transformer.
My preamp supply section layout is something like this:
An externally hosted image should be here but it was not working when we last tested it.
I just connected Amp+ and Amp- to the AC connection there (previously).
Coincidentally, the power supply is running 10mA through the zeners, which is slightly less than my original design, and it's running perfectly. And the supply capacitors are only 470uF, and a further 100uF on the zeners (and obviously the decoupling capacitors).
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So here's the power amplifier design. I will post the preamplifier design sometime.
I would use this amplifier with only the +-26 V rail specification. There is no need to try different voltages, and other amplifiers can be built for different supplies. This supply specification is nice, because it gives decent power, doesn't require a very big transformer, and is also fairly low voltage. I had the transformer, and that's why I designed the amplifier.
The amplifier is designed to work with a preamplifier. The gain and input impedance are not sufficient for this to be used as a stand-alone amplifier.
Design notes:
I designed the LTP to operate at 1 mA per side; this biases the transistors well (and it's easy to select resistor values for this). The amplifier stage is designed to have no problem driving the output stage for any variations in load impedance for a 4 ohm nominal load. This amplifier will drive 2 ohms, but there is a risk of blowing the output transistors. If you intend to build this, and drive 2 ohms, use TIP35C/TIP36C rather than 41/42.
I have no measurements yet, but they will come in good time (but don't expect THD percentages - I don't have the equipment).
T3 must rather be a 2SA1837.
2N5551's can be changed for BC547C's if you would like. I've grown fond of the 2N5551/2N5401's, so I've started to use them in all my designs.
A 220 pF capacitor can be connected from base to collector on T4 and one on T5. All 220 pF capacitors can probably be decreased to 100 pF. I don't see any reason for or against this. Maybe those more experienced will have reasons for or against this. I just like the larger capacitance. The 220 pF theoretically shouldn't mess around with the audible frequencies.
All resistors are 1 % 0.25 W, except those shown with different power. This amplifier allows for a lot of tolerance. The only thing is that the better the tolerance, the better the CMRR, but I don't suspect anyone will notice the difference between 1 % and even 10 % resistors.
The diode can be replaced with a biasing circuit, consisting of a suitable NPN transistor (ideally BC547), a resistor and a trimmer. I would use a 1k from collector to base and a 5k trimmer (10 turn) from base to emitter. With just the diode, I can't hear the crossover distortion, but I will measure what it looks like. As long as the biasing diode/transistor is thermally coupled to T4 or T5.
This amplifier was designed for a bridged design, but with the TIP41/2 transistors, this will not be advisable. I'm using it as a 4 channel amplifier for bi-amplification.
I would recommend a 2.5 K/W heatsink per amplifier. I'm using 0.7 K/W for two. I also recommend a small heatsink on T3.
Measured so far, the DC on the output is 30 mV.
I will post the PCB for this soon.
I would use this amplifier with only the +-26 V rail specification. There is no need to try different voltages, and other amplifiers can be built for different supplies. This supply specification is nice, because it gives decent power, doesn't require a very big transformer, and is also fairly low voltage. I had the transformer, and that's why I designed the amplifier.
The amplifier is designed to work with a preamplifier. The gain and input impedance are not sufficient for this to be used as a stand-alone amplifier.
Design notes:
I designed the LTP to operate at 1 mA per side; this biases the transistors well (and it's easy to select resistor values for this). The amplifier stage is designed to have no problem driving the output stage for any variations in load impedance for a 4 ohm nominal load. This amplifier will drive 2 ohms, but there is a risk of blowing the output transistors. If you intend to build this, and drive 2 ohms, use TIP35C/TIP36C rather than 41/42.
I have no measurements yet, but they will come in good time (but don't expect THD percentages - I don't have the equipment).
T3 must rather be a 2SA1837.
2N5551's can be changed for BC547C's if you would like. I've grown fond of the 2N5551/2N5401's, so I've started to use them in all my designs.
A 220 pF capacitor can be connected from base to collector on T4 and one on T5. All 220 pF capacitors can probably be decreased to 100 pF. I don't see any reason for or against this. Maybe those more experienced will have reasons for or against this. I just like the larger capacitance. The 220 pF theoretically shouldn't mess around with the audible frequencies.
All resistors are 1 % 0.25 W, except those shown with different power. This amplifier allows for a lot of tolerance. The only thing is that the better the tolerance, the better the CMRR, but I don't suspect anyone will notice the difference between 1 % and even 10 % resistors.
The diode can be replaced with a biasing circuit, consisting of a suitable NPN transistor (ideally BC547), a resistor and a trimmer. I would use a 1k from collector to base and a 5k trimmer (10 turn) from base to emitter. With just the diode, I can't hear the crossover distortion, but I will measure what it looks like. As long as the biasing diode/transistor is thermally coupled to T4 or T5.
This amplifier was designed for a bridged design, but with the TIP41/2 transistors, this will not be advisable. I'm using it as a 4 channel amplifier for bi-amplification.
I would recommend a 2.5 K/W heatsink per amplifier. I'm using 0.7 K/W for two. I also recommend a small heatsink on T3.
Measured so far, the DC on the output is 30 mV.
I will post the PCB for this soon.
Here is the preamplifier. It will drive 2 amplifiers easily for full range, giving the 4 channels. It is a simple class A preamplifier. I don't have a PCB for this because the PCB I used was for a bridged amplifier, so I used an inverting amplifier to drive the second one. I modified this on that PCB. I do have a PCB for the preamplifier without the active crossover section.
From Vin to Vout is the preamplifier. C3 is included. The power supply must be regulated for best performance.
Current draw is only about 4 - 5 mA per preamplifier.
There is an offset of about -300 mV at Vout.
If you need to change the crossover point, C4 and C5 must be the only components which change. R9 and R10 are important.
This preamplifier was designed to drive the power amplifier shown above, and not as a stand-alone preamplifier. It will drive a 5k ohm load, and can be used as a stand-alone unit, but it will not give good technical performance. Theoretically, it should have a THD figure in the region of 0.01%, however, it cannot have crossover distortion, so the distortion is "pure".
From Vin to Vout is the preamplifier. C3 is included. The power supply must be regulated for best performance.
Current draw is only about 4 - 5 mA per preamplifier.
There is an offset of about -300 mV at Vout.
If you need to change the crossover point, C4 and C5 must be the only components which change. R9 and R10 are important.
This preamplifier was designed to drive the power amplifier shown above, and not as a stand-alone preamplifier. It will drive a 5k ohm load, and can be used as a stand-alone unit, but it will not give good technical performance. Theoretically, it should have a THD figure in the region of 0.01%, however, it cannot have crossover distortion, so the distortion is "pure".
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This system is not high quality by the standards of a lot I've read on the forums, but it is easy to build, and it sounds great to my ears. It is very stable - in other words, it was designed to be stable, and I haven't had any issues at all so far. And, importantly, it is cheap.
R4 in the power amplifier schematic can be replaced with a 2 mA current source, and this should make it a very high performer. It is worth it because it's an easy change and a cheap one.
As I've said, BC547's (and BC557 for the PNP in the preamp) can be used, and will probably give slightly better performance. I just stock up on (and like) the 2N5551/2N5401's.
R4 in the power amplifier schematic can be replaced with a 2 mA current source, and this should make it a very high performer. It is worth it because it's an easy change and a cheap one.
As I've said, BC547's (and BC557 for the PNP in the preamp) can be used, and will probably give slightly better performance. I just stock up on (and like) the 2N5551/2N5401's.
The combined gain of the pre-amp and the power amp is 10k/5k6+1 * 12k/1k+1 = 36.2times (+31.2dB)
The maximum output is roughly 20Vpk or 14Vac.
The maximum input at the pre-amp will be around 387mVac
You are planning far too much gain for most sources.
Through away the pre-amp and use a Buffer instead.
The makes your input sensitivity ~ 1077mVac.
Most Sources exceed this and some by more than 6dB.
The maximum output is roughly 20Vpk or 14Vac.
The maximum input at the pre-amp will be around 387mVac
You are planning far too much gain for most sources.
Through away the pre-amp and use a Buffer instead.
The makes your input sensitivity ~ 1077mVac.
Most Sources exceed this and some by more than 6dB.
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Yes, it is a lot of gain overall. It will require 0.7 Vpeak for 20 V output peak, but I'm aiming for 25 V. The supply rails will be more than 26 V with little to no load (measured), and, as you know, I can aim for the higher voltage rails because the larger transients will use that voltage stored up. I'm more aiming for 0.9 Vpeak. For my source, this is perfect. I control my volume remotely from the source, and it barely reaches 1 Vac.
Thanks for the input! I appreciate the ideas there.
ETA: The filters are designed to eat a bit of gain - about 15%, and that's why 0.7 V for 20 V output.
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you can't get 25Vpk from a amplifier fed with ±26Vdc
and that leaves you with a power amp sensitivity for maximum power of around 14Vac/13times = 1077mVac
Plenty for most sources and far too much for some sources.
For the majority of Sources you don't need a pre-amp.
Have you got a source that has a very low maximum output?
A reasonable maximum is around 14Vac to 15Vac.
and that leaves you with a power amp sensitivity for maximum power of around 14Vac/13times = 1077mVac
Plenty for most sources and far too much for some sources.
For the majority of Sources you don't need a pre-amp.
Have you got a source that has a very low maximum output?
You are so right. However, the transformer is 18-0-18 Vac at 230 Vac primary and it's a 225 VA. Our line is 240 Vac, but I calculate the DC as 18 * 1.42 = 26 (rounded up). That's the specification I use. It can go up and down by a few without changing the bias points. Furthermore, the transformer will output higher at no load. Then, I have a 2.2k in series with my input impedance (about 12k), which makes a divider, and lowers overall gain to about 28 V/V. I've measured it at no load now, and I can't get it to clip. Full swing was at 25.x Vpeak, and it was still 100% clean. I haven't had time to get results yet, I just set up my scope and did a quick test. I'll do more later and post.
On the scope, I've gotten an output of 23 V peak, and that 23 V also makes it to the load. I haven't been able to get the amp to clip, even with a load. It goes down to 20 Hz without cutting too much. I can't test the high end because I don't have a generator with the capability. With a load, crossover distortion comes in at low levels and higher frequencies, but is really very little. Not enough for me to make the effort of building and setting 4 bias circuits.
0.2V/div:
5V/div, -5V offset:
Similar results with an 8 ohm load.
The frequency is around 450 Hz.
5V/div, -5V offset:
Similar results with an 8 ohm load.
The frequency is around 450 Hz.
Oh, and an important note on my preamplifier - the output of the op amp must go to the HF amplifier via a 100 or 150 ohm resistor.
I did a listening test, biamped my speakers. Very satisfactory indeed! I can hear a tiny bit of crossover distortion, but it's really not noticeable. May it be sounded like crystal. Crossover distortion is only really noticeable at extremely low levels on high vocal music with your ear about a foot from the speaker. I might bias the amplifiers a bit before final construction, but i don't think it's necessary. If the amp was for someone else, it wouldn't be a question.
So the verdict on biamplification is that it really does provide a lot of extra headroom. The SPL is similar to that of a high power amplifier, but it doesn't sound like the speakers are straining as much, if that makes sense. Can't wait to build it into an enclosure.
So the verdict on biamplification is that it really does provide a lot of extra headroom. The SPL is similar to that of a high power amplifier, but it doesn't sound like the speakers are straining as much, if that makes sense. Can't wait to build it into an enclosure.
Enclosure is built. I just need to cut the top panel vents.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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