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3 Transistor HP Amplifier with low dist
3 Transistor HP Amplifier with low dist
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Old 19th September 2020, 11:58 AM   #11
avtech23 is offline avtech23  Australia
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3 Transistor HP Amplifier with low dist
Hi Marin,

Spice shows R1 and R6 at 288mW, R8 at 194mW, so 0.6w probably ok but maybe a bit warm.

The BD139 is showing around 300mW, so I would put a little heatsink on that for sure.

Clipping comes in around 550mV.
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Old 22nd September 2020, 06:43 AM   #12
avtech23 is offline avtech23  Australia
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3 Transistor HP Amplifier with low dist
Finally got a chance to knock up a prototype of this one:

3 Transistor HP Amplifier with low dist-20200922_152533-jpg


No smoke or fire on power up, which is good!

However, the circuit oscillates as soon as it is turned on.
Attached Images
File Type: jpg 20200922_152533.jpg (697.7 KB, 974 views)
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Old 23rd September 2020, 12:27 AM   #13
avtech23 is offline avtech23  Australia
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3 Transistor HP Amplifier with low dist
Oscillation seems to have been tamed using a 100pF compensation cap across the BC560. I tried 33pF but that wasn't sufficient - 100pF was next in stock.

I added 100n at the input to filter out a pulsing noise that I could hear, and now it is nice and quiet.

I used 600mW resistors and the 33 ohm are warm (60 deg) at 12 degrees ambient. The BD139 was about the same temperature without heatsinking.

Clipping is at 1.9v across a 47 ohm load resistor (I didn't have anything closer to 32 ohm for testing).

Sounds pretty good on the first test! I'll try to get some measurements done soon.
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Old 24th September 2020, 07:16 PM   #14
NealJ is offline NealJ  United Kingdom
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I'll try to get some measurements done soon.

Looking forward to these, thanks for posting your progress so far.
A beginner's question if I may? Does the oscillation come from the circuit as is or is it due to the layout or some other factor?
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Old 25th September 2020, 04:10 AM   #15
avtech23 is offline avtech23  Australia
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3 Transistor HP Amplifier with low dist
Not sure to be honest. I'm certainly no layout expert so it is entirely possible I've committed some rookie error.

I'll try and do a phase margin plot on LTspice (when I remember how) and see if anything shows up there.

Here is the latest iteration of the board:

3 Transistor HP Amplifier with low dist-pcb-jpg
Attached Images
File Type: jpg PCB.JPG (80.1 KB, 978 views)
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Old 2nd October 2020, 02:06 PM   #16
avtech23 is offline avtech23  Australia
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3 Transistor HP Amplifier with low dist
I made the above version of the board using 1W resistors for the 33R and 22R positions. (Note - I did not build the PSU filter, just the amp, so switching noise is possibly present).

I changed R4 and R3 to 180k and 51k respectively as these values are easier to obtain than the 191k. I simulated in LT and the change seemed to be suitable.

So here are the measurements into 47 Ohms, using a Focusrite Scarlett 2i4, laptop on battery:

3 Transistor HP Amplifier with low dist-results-png

THD:
3 Transistor HP Amplifier with low dist-spectrum_thd-png

Noise:
3 Transistor HP Amplifier with low dist-spectrum_noise-png

Frequency Response:
3 Transistor HP Amplifier with low dist-spectrum_fr-png
Attached Images
File Type: png Results.PNG (41.2 KB, 924 views)
File Type: png Spectrum_thd.png (108.4 KB, 923 views)
File Type: png Spectrum_noise.png (95.5 KB, 926 views)
File Type: png Spectrum_FR.png (53.5 KB, 936 views)
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Last edited by avtech23; 2nd October 2020 at 02:11 PM.
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Old 3rd October 2020, 01:36 PM   #17
avtech23 is offline avtech23  Australia
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3 Transistor HP Amplifier with low dist
Ok I seem to have taken up half of this thread, so last one from me.

I've made some measurements with a CRC filter at the front of the power supply and got the following:

3 Transistor HP Amplifier with low dist-results-png

THD:
3 Transistor HP Amplifier with low dist-spectrum_thd-png

Noise:
3 Transistor HP Amplifier with low dist-spectrum_noise-png

Frequency Response:
3 Transistor HP Amplifier with low dist-spectrum_fr-png



Attached Images
File Type: png results.PNG (28.1 KB, 905 views)
File Type: png Spectrum_FR.png (51.8 KB, 905 views)
File Type: png Spectrum_Noise.png (97.4 KB, 908 views)
File Type: png Spectrum_THD.png (97.1 KB, 904 views)
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Old 4th October 2020, 10:09 PM   #18
sgrossklass is offline sgrossklass  Germany
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1. The 2-3 kHz artifact is probably ground loop noise. Make a cable that connects unbal-signal --> hot and unbal-ground --> cold, with shield connecting on the interface input side only. Then the balanced input should also be operating as such for once. (I mean, I could also be wrong and the noise may be inherent to the interface. Then it would also appear in a direct loopback.)

2. There is something amiss with the sample rate, the interface seems to run at 44.1 kHz only. If switching to ASIO in RMAA does not fix this, further investigation will be necessary (check interface control panel, check for updated drivers etc.).

3. Most any half-decent multimeter should be suitable for absolute level calibration, assuming it has something like a 20Vac range at least. Use 50-60 Hz if not True RMS.

Last edited by sgrossklass; 4th October 2020 at 10:13 PM.
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Old 7th October 2020, 01:52 AM   #19
avtech23 is offline avtech23  Australia
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3 Transistor HP Amplifier with low dist
Thanks for the tips.

I revisited the cables and they have been made correctly - twin shielded and screened audio cable with one wire from to signal - signal, one from gnd - gnd and shield connected to gnd only on the interface side.

Switching to ASIO makes the DUT clip so I need to look into this more.

I've had a look at the absolute level calibration and have a Fluke179 that is capable. I'll have a play with this to get consistency.

What voltages do you suggest for the testing?
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Old 15th October 2020, 05:47 PM   #20
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by avtech23 View Post
Thanks for the tips.

I revisited the cables and they have been made correctly - twin shielded and screened audio cable with one wire from to signal - signal, one from gnd - gnd and shield connected to gnd only on the interface side.
I am not actually convinced that this is correct. You should have the following input connector pinout:
(XLR | TRS - at cable - at DUT output)
pin 1 | sleeve - cable shield - n/c
pin 2 | tip - cable hot - signal
pin 3 | ring - cable cold - gnd

Here's a matching attenuator topology as well:
3 Transistor HP Amplifier with low dist-spk2xlr-png
This is useful for both BTL and SE outputs.

You may not need one if you have cables both for XLR (mic) and TRS (lone-level) input, as it looks like you have a wide variety of maximum input levels to choose from:
+3.5 dBu - XLR, no pad (there should be little degradation in input dynamic range when turning the gain up by up to about 20 dB)
+13.5 dBu - XLR, w/ pad
~+24 dBu - TRS, I'm guessing w/ pad
(Values taken from 2i4 1st gen manual, assuming this is the one you have.)

Quote:
Originally Posted by avtech23 View Post
Switching to ASIO makes the DUT clip so I need to look into this more.
You may have had the output turned down digitally (maybe the software volumee control exposed in the OS is driver-level attenuation only). Turn down the 2i4's MONITOR control to reach previous levels. Turning up volume in Windows should bring non-ASIO levels up.
Quote:
Originally Posted by avtech23 View Post
I've had a look at the absolute level calibration and have a Fluke179 that is capable. I'll have a play with this to get consistency.

What voltages do you suggest for the testing?
1 Vrms (XLR no pad) and 2 Vrms (XLR w/ pad) would be pretty standard, I suppose.
4 Vrms would be of interest with a 300 ohm load (TRS input should still do that w/o pad)
400 mV may be of interest with e.g. a 32 ohm load in particular (I presume distortion with 300 ohms would be well-described by intercept points at these levels, i.e. should agree well with levels extrapolated from 1 Vrms).
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File Type: png spk2xlr.png (5.6 KB, 799 views)
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