audiosource amp one A, biasing voltage question - Page 1
Forum section: electronics / repair

author: ceramic
author_rank: Newbie
timestamp: June 27, 2025, 07:12:54 pm
content: hello everyone,i am about to recap an amplifier after the caps tested poorly and because i do not seem to be able to find the repair manual i do not know what the bias voltage should be.is it possible to deduct it or at least someone w/ experience in amp repair could offer a suggestion?as of now w/ faulty caps the bias is around 4 and 7 mV for the two channels. the amp uses toshiba C5198 and A1941, 50V rails.is anyone familiar w/ this amp or experienced in amp repair to offer me advice?thank you for your time, have a great day

author: edavid
author_rank: Super Contributor
timestamp: June 27, 2025, 07:59:09 pm
content: Quote from: ceramic on June 27, 2025, 07:12:54 pmIf you are only changing caps, you shouldn't need to re-adjust the bias voltage.QuoteThat can't be right for base emitter voltage. What are you measuring?If it's across an emitter resistor, what is the resistor value?Do you have access to a distortion analyzer?

author: ceramic
author_rank: Newbie
timestamp: June 27, 2025, 10:33:14 pm
content: hi and thank you.the board appears to have two polls per channel for adjusting bias, connected to the Es of the transistors, i measured between those. will attach pic.the power supply caps were so dry, one measured over 50ohms esr affecting the rail voltage, one measured around 43VDC the other 47 i guess they're meant to be 50... other caps were terrible too and i suspect were influencing all kind voltages because the speaker relay takes about a min to engage as the voltage builds up for its coil. the new caps arrive today but i did swap a few caps a few days ago w/ something i had around and the speaker relay engaged in a timely manner.by the amount of dust i found inside i suspect this amp was used as a zone 2 amp, always on. anyways, thank you for helping me.

author: temperance
author_rank: Super Contributor
timestamp: June 27, 2025, 10:55:52 pm
content: QuoteIf that is 7 mV per emitterresistorand note base emitter voltage, this equates into about 21 mA quiescent current for each transistor. That seems fine to me.

author: edavid
author_rank: Super Contributor
timestamp: June 27, 2025, 11:34:00 pm
content: Quote from: temperance on June 27, 2025, 10:55:52 pmAgreed. I wouldn't mess with it.

author: ceramic
author_rank: Newbie
timestamp: June 28, 2025, 09:32:49 am
content: thank you guys, i appreciate your help.i have replaced the capacitors and measured some voltages. there is a slight disparity between the measurements of the 2 channels.with the amp idling, heatsinks ~37C, voltages stable (after 10-15 minutes) i recorded these readings:1. speaker terminals L=5.1mVdc, R=7.4mVdc2. across the polls shown in previous post (emitter of first transistor to emitter of 4th transistor)channel 1 = 19mVdcchannel 2 = 17mVdc3. across the 0.33ohm resistor connected to the emitter of transistor (the polls are connected to these resistors):channel 1resistor connected to emitter of C5198 = 9.6mVdcresistor connected to emitter of A1941 = 9.3mvdcchannel 2resistor 1 = 8.3mVdcresistor 2 = 8.1mVdci figured from your replies the biasing refers to the amount of current flowing thru the transistors at idle and it should be somewhere around 20mA.i calculated (based on the voltage drop across the 0.33 ohm resistors connected to the transistors' emitters) the values of 28mA for channel 1 and 24mA for channel 2. (pls correct me if i misunderstood as i have limited experience/knowledge w/ electronics)i am including a few pics for clarification purposes if my methodology was confusing. the probes are blue and brown.

author: temperance
author_rank: Super Contributor
timestamp: June 28, 2025, 01:16:47 pm
content: There is no need to worry about the bias adjustment and I would leave it as it is. Small differences in voltage across individual emitter resistors is perfectly normal and the reason why those resistors there.

author: ceramic
author_rank: Newbie
timestamp: June 28, 2025, 04:37:41 pm
content: i understand, thank you for your advice. so everything regarding the bias looks satisfactory i could put some load on the amp...may i ask another question for educational purposes: the AC out of the transformer measures 38Vac (between one lead and the ground, same for the other as 3wires come out of it) but the voltage across the power supply caps measures 52VDC (the caps are 50V/10000uF as the originals, i picked this one (link:https://www.mouser.com/ProductDetail/232-10000MEFCSN30X40).how does that happen for the caps to charge up higher than the input voltage and it is it safe for the proper functioning of the capacitors to be 2Vdc above the max specified voltage?

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2025, 04:52:12 pm
content: The peak voltage of a sine wave is 1.41 times the RMS voltage.AC voltmeters are normally calibrated for RMS, but the capacitor will charge to the peak voltage, but droop somewhat between current pulses through the diodes (two per sine-wave period).

author: ceramic
author_rank: Newbie
timestamp: June 28, 2025, 05:43:35 pm
content: thank you mr Tim for the explanation