Hello All,
Got this little amp to use with my Sennheiser HD599 headphones.
It came ready built and I just had to provide it with a proper power transformer and a power supply (using an LM337 reg at 20 VDC).
Will provide more details on the actual PSU I am using, in a following post.
The transistors are Toshiba PNP 2SB292 (https://www.web-bcs.com/transistor/tc/2sb/2SB292_cpl.php) and the output transformers are rated for 32Ohm.
Here is the schematic:
It seems that the 200kOhm resistor in parallel with the 200kOhm pot sets the bias on the device.
However, I am completely clueless on what to look for to set the right bias for this circuit.
Any help regarding proper bias, will be appreciated !!
Pics of the actual board with its output transformers:
The amplifier is working OK. Not any noticeable hum or noise.
However, compared to other headphone amps of mine (Lehmann BCL clone, OTL tube amp by Aren van Waarde, Zen-style amp by Tortello and other) it sounds pretty thin. It lacks beef at the lower end of the spectrum.
Otherwise it sounds OK mid-band and higher up.
So, I wonder if there is something to look for, in order to improve things there.
Thanks a lot for reading!
Nick.
Got this little amp to use with my Sennheiser HD599 headphones.
It came ready built and I just had to provide it with a proper power transformer and a power supply (using an LM337 reg at 20 VDC).
Will provide more details on the actual PSU I am using, in a following post.
The transistors are Toshiba PNP 2SB292 (https://www.web-bcs.com/transistor/tc/2sb/2SB292_cpl.php) and the output transformers are rated for 32Ohm.
Here is the schematic:
It seems that the 200kOhm resistor in parallel with the 200kOhm pot sets the bias on the device.
However, I am completely clueless on what to look for to set the right bias for this circuit.
Any help regarding proper bias, will be appreciated !!
Pics of the actual board with its output transformers:
The amplifier is working OK. Not any noticeable hum or noise.
However, compared to other headphone amps of mine (Lehmann BCL clone, OTL tube amp by Aren van Waarde, Zen-style amp by Tortello and other) it sounds pretty thin. It lacks beef at the lower end of the spectrum.
Otherwise it sounds OK mid-band and higher up.
So, I wonder if there is something to look for, in order to improve things there.
Thanks a lot for reading!
Nick.
I do not think that 200k potentiometer is drawn correctly. No matter how its adjusted, its practically shorted, bypassed.
A bunch of capacitors isnt going to do much on a regulated supply. The impedance of just ONE of those capacitors is low enough compared to 600 ohms that “beef” shouldn’t be a problem. Output transformers, especially single-ended ones, have a low frequency limit. If there is not enough primary inductance, bass suffers. If the bias is too high, inductance will drop too - so turning up the bias has the opposite effect that one might think.
Good that you already have some headphone amps. Perhaps save that 1 transistor thing for a rainy day in a leap year. 🙂
Looks like there is a polarized cap C7 on the input. It might cause problems such as loading of the source. I think that array of filter caps is only for fitting the PCB in a small case.
Target the 30 mA, not the 10 volts. The voltage you end up with will depend on the DCR of the transformer. Centering the voltage at the midpoint would only be correct if the DCR is about 300 ohms. If the DCR is lower (probably is) the voltage at the collector will be closer to the rail. If a “600 ohm” transformer really had 300 ohms at DC that would be one crappy transformer and the resulting amplifier probably wouldn’t sound very good.
Ok, that transistor has a .150 mW power limit. Given class A operation with a 20 volt supply (assumes minimal dc voltage drop of the transformer) the max bias current would be 7.5 mA. Given the large heat sink, that might be pushed a bit 10 mA maybe???
Also the max Vcb is 30 volts. With a transformer load its possible to drive the collector to nearly 2x the supply voltage or 40 volts. If this were mine I would reduce the supply to 15 volts and bias at 10 mA. That should keep the transistor within its safe operating limits.
Also the max Vcb is 30 volts. With a transformer load its possible to drive the collector to nearly 2x the supply voltage or 40 volts. If this were mine I would reduce the supply to 15 volts and bias at 10 mA. That should keep the transistor within its safe operating limits.
Item: https://www.aliexpress.com/item/3256803198397941.html
The bias pot better not be wired like that or instant smoke. It will still smoke if carelessly turned to zero. Again I hope it is not wired like that... perhaps the fixed is series to the pot?
It's a dangerous bias technique. Base current is fixed by base resistor/pot. Collector current is base current time hFE. Fine. Except hFE will typically rise with temperature. Making more collector current, and more heat. Emitter resistor does NOT stabilize bias in this circuit. Therefore it wants to run-away.
Since the aim is 600 Ohms load, 10V 15mA looks like a best-fit. (647r total, 9.85V 15.2mA)
While late-production parts sometimes had more guts than the original datasheet, it isn't a good bet.
It says right on it, 30mA. So figure what 30mA is in the 47 Ohm emitter resistor. 1.41 Volts. Trim for that much. (You could figure from transformer resistance but is best to go by a Real Resistor.)
The bias pot better not be wired like that or instant smoke. It will still smoke if carelessly turned to zero. Again I hope it is not wired like that... perhaps the fixed is series to the pot?
It's a dangerous bias technique. Base current is fixed by base resistor/pot. Collector current is base current time hFE. Fine. Except hFE will typically rise with temperature. Making more collector current, and more heat. Emitter resistor does NOT stabilize bias in this circuit. Therefore it wants to run-away.
Dang, you are right. https://transistordata.com/bjt/2sb292
Since the aim is 600 Ohms load, 10V 15mA looks like a best-fit. (647r total, 9.85V 15.2mA)
While late-production parts sometimes had more guts than the original datasheet, it isn't a good bet.
Thank you all very much for all your valuable input. I will try your suggestions regarding bias and power VDC.
Since i didn't provide much details of the actual output transformers, here is a picture showing impedance and inductance:
You might have some thoughts, regarding their performance in the lower end of the frequency spectrum.
The power sypply is the usual LM337 affair with adjustable output VDC using a pot.
Since i didn't provide much details of the actual output transformers, here is a picture showing impedance and inductance:
You might have some thoughts, regarding their performance in the lower end of the frequency spectrum.
The power sypply is the usual LM337 affair with adjustable output VDC using a pot.