3 Transistor HP Amplifier with low dist

Formula3HP

Here is an Amplifier using 3 transistors.
It has low distortion.

Gain is x4 which should be good for 32 Ohm headphones.
Power supply could be one LM317 set at 12 VDC.
All resistors can be 0.6 Watt.

Formula3HP
 

Attachments

  • HP-Special-2-Pub3.jpg
    HP-Special-2-Pub3.jpg
    103.2 KB · Views: 4,148
Last edited:
Looks a nice simple single ended design, and I have almost all the required transistors!

missing the BC550C

Could I ask, what is the maximum input signal voltage?

Or put another way, when does clipping occur when the input signal is raised to 2.5Vp-p, for example?

Maybe 3dB of excess gain, for some situations. So perhaps room for a low level of NFB too, if desired

I'm going to find some BC550C, and build it!
 
Last edited:
Maybe 3dB of excess gain, for some situations. So perhaps room for a low level of NFB too, if desired

Don't build it if you're leery of NFB -- it already has plenty.;)

The gain is set by the divider R1, R6, R8. If you want to cut 8 dBV of gain, just move the pick-off for U1's Emitter to the node between R1 and R6. There should be enough spare bias current that you won't have to fiddle U1's bias divider.

Nifty little design. Congrats lineup!:cheers:

Cheers
 
Cheers Rick,

It isn't obvious to me that there is NFB, other than local degeneration. But I'm no expert.
I completely appreciate being made fo feel retarded, thank you!

PRR,

Funnily enough I do have a volume control, and even funnier, I usually design any amplifier to have the minimum excess gain, or just enough gain to do the job.

Is that wrong, when you know the outputs of your sources?

E.g. 32R phones dont need 8V p-p with 2V p-p signal input, unless the source is non standard output level or the phones are hideously insensitive.

But, Lineup circuit is elegant in its simplicity, and I cant wait to build, test, and modify. See how it performs in reality.
 
Last edited:
Lineup - I hope you don't mind but just for fun I threw together a board based on your design.

It's about 80*60mm (can easily be made smaller).

I intend on using a decently regulated PSU so I added a CRC filter (values to be finalised), instead of LM317. The CRC could also be CLC or just bypassed.

Can anyone see anything critically wrong or anything that stands out as might be problematic?

This layout is just single sided so the power supply is a little awkward being asymmetrical but I think it should be ok for a low power circuit.

attachment.php


attachment.php


attachment.php


attachment.php


attachment.php


I've only designed a few boards so far, so I'm sure there are plenty of technique errors or misplaced components! :eek:

Cheers.
 

Attachments

  • Board3d1.PNG
    Board3d1.PNG
    95.7 KB · Views: 3,870
  • Board3d2.PNG
    Board3d2.PNG
    46.3 KB · Views: 3,877
  • Board3d3.PNG
    Board3d3.PNG
    52.1 KB · Views: 3,910
  • Board Bare.PNG
    Board Bare.PNG
    103.7 KB · Views: 3,921
  • Schematic.PNG
    Schematic.PNG
    65 KB · Views: 3,950
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.
 
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:

attachment.php
 

Attachments

  • PCB.JPG
    PCB.JPG
    80.1 KB · Views: 2,219
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:

attachment.php


THD:
attachment.php


Noise:
attachment.php


Frequency Response:
attachment.php
 

Attachments

  • Spectrum_FR.png
    Spectrum_FR.png
    53.5 KB · Views: 2,100
  • Spectrum_noise.png
    Spectrum_noise.png
    95.5 KB · Views: 2,077
  • Spectrum_thd.png
    Spectrum_thd.png
    108.4 KB · Views: 2,094
  • Results.PNG
    Results.PNG
    41.2 KB · Views: 2,094
Last edited:
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:

attachment.php


THD:
attachment.php


Noise:
attachment.php


Frequency Response:
attachment.php




:Pawprint:
 

Attachments

  • results.PNG
    results.PNG
    28.1 KB · Views: 2,047
  • Spectrum_FR.png
    Spectrum_FR.png
    51.8 KB · Views: 2,041
  • Spectrum_Noise.png
    Spectrum_Noise.png
    97.4 KB · Views: 2,070
  • Spectrum_THD.png
    Spectrum_THD.png
    97.1 KB · Views: 2,046
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:
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?
 
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:
attachment.php

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.)

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.
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).
 

Attachments

  • spk2xlr.png
    spk2xlr.png
    5.6 KB · Views: 1,925