No hiss on discreet "preamp" when volume at max and bias - Page 1
Forum section: electronics / repair

author: nuno
author_rank: Frequent Contributor
timestamp: November 26, 2020, 08:46:57 pm
content: Hello all,I have this behavior in a discreet old "pre-amp" (a 2-transístor amplifier with a "hi cut" mechanical switch, before a passive tone control stage), that has that typical hiss (white) noise except when the volume is at maximum (volume pot at the amp's input). I can ear this clearly when no input is connected. There's audio at any volume level (volume depending on pot setting, as normal). Is this some kind of well-known defect?Another thing I would like to ask is, when you see an idle voltage marked on a schematic for an amp, how much measured value deviation percent is acceptable? Lets say it's marked 4.5V.Thank you!

author: bob91343
author_rank: Super Contributor
timestamp: November 26, 2020, 10:49:35 pm
content: Tolerance varies depending on the circuit but in general 20% is usually okay. The purpose of measuring volts is to track down problems and if there is a major problem the voltages will be very far from marked.As for the hiss, perhaps checking with an oscilloscope would be a good idea. It may be that it's still there but centered at a frequency too high to hear. If the unit works, leave it alone.

author: WD40
author_rank: Contributor
timestamp: November 27, 2020, 11:22:23 am
content: When the pot's at maximum, it's effectively bypassed - creating a short. When it's below maximum it's introducing series resistance, which equals hiss. The higher the source resistance, the higher the noise - simple. The way around this is to use an active volume control, where instead of acting as a voltage divider, the pot is in the feedback loop, varying gain with position. In this scenario, you get maximum hiss when the pot's at max. In a typical audio system that has several components, you will find that designers mix passive and active level controls, depending on application. The lower the value of the pot, the less noise when wired as a divider. But this comes at a price of higher loading for the circuit driving it. A 1K pot will have a lot less noise wired passively than 100K, but not all opamps will be happy to drive it.

author: nuno
author_rank: Frequent Contributor
timestamp: December 16, 2020, 05:11:08 pm
content: Well, I am unable to scope the noise, as soon as I connect my scope's ground to the amp's ground it injects a strong tone.... the device, being old, isn't grounded.I have another noise issue... please see the attached recording. It has 3 different noises that stand out, 1) the mains 50Hz hum, 2) a white noise and 3) another kind of variable not so white noise, our culprit. Let's ignore the 1st 2 noises. This 3rd noise happens only in one of the channels of a transistorized (3 BJT) phono amp. It's there even with the input grounded. Does anyone have hints or knows possible causes of this kind of noise? Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2020, 05:32:06 pm
content: Quote from: WD40 on November 27, 2020, 11:22:23 amTo expand further, the reason for the increased noise, at higher input resistances is the amplifier will have a certain input current noise, which is multiplied by the input resistance's value, to form a voltage. The input current noise comes from the fluctuation in bias current. For example if your amplifier is a simple BJT amplifier, the base current will fluctuate randomly, over time, which will present a fluctuating load to the circuit driving it. The higher the output resistance of the circuit driving your amplifier, the greater the fluctuation in voltage.Resistors also generate thermal noise, but this is normally only significant for FET input amplifiers, connected to a high input impedance.Quote from: nuno on December 16, 2020, 05:11:08 pmIt sounds like the oscilloscope is coupling some mains hum into the circuit.Sorry I can only hear mains hum and a crackle. Is it a high frequency, which I might not be able to hear? I'll try again later with better speakers. Perhaps it's oscillating.Please post a schematic.

author: nuno
author_rank: Frequent Contributor
timestamp: December 16, 2020, 05:41:47 pm
content: Yes, it's a kind of crackle. Schematics in attach. ThanksQuote from: Zero999Doesn't seem to be mains hum, it sounds quite squarish, nothing like hum.I'll look more carefully into your explanation, thanks.

author: nuno
author_rank: Frequent Contributor
timestamp: December 16, 2020, 05:45:54 pm
content: Quote from: Zero999 on December 16, 2020, 05:32:06 pmSo the question is: is this normal behavior (no noise at max volume, noise below that) or a defect with the amp's bias circuit? I know it's an old device (1972, I think), but... sounds relatively high volume. The volume pot is 500K and is one of those 4 tap used with loudness. With the 4th tap connected, it creates a no-noise zone (noise = white noise) at 40% - 50% pot volume. No noise only at max volume is without the 4th tap connected.

author: Datman
author_rank: Regular Contributor
timestamp: December 16, 2020, 05:57:43 pm
content: Without a low impedance source connected at the input, noise is surely high.Where is the potentiometer? The schematic is only of the RIAA phono preamp.

author: nuno
author_rank: Frequent Contributor
timestamp: December 16, 2020, 06:14:01 pm
content: Ah, right.... the volume pot is at the output of the RIAA amp (in fact, there's a 500K NM balance pot, then 500K volume with loudness tap pot).

author: nuno
author_rank: Frequent Contributor
timestamp: December 16, 2020, 06:16:12 pm
content: Quote from: Datman on December 16, 2020, 05:57:43 pmOk, I was using a rather high impedance (400K+) input source, since I don't have the phono system. But the volume pot white noise happens on other sources too (RIAA amp out of the way). But maybe it's the cause of the crackle.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 17, 2020, 01:25:45 pm
content: Quote from: nuno on December 16, 2020, 06:14:01 pmThat's the problem. If you get us the full schematic, we'll be able to determine if it might be possible to replace it with a lower value one. (The preamp you showed will be OK with a 50k if you disconnect its output resistor to ground.)

author: nuno
author_rank: Frequent Contributor
timestamp: December 17, 2020, 03:13:06 pm
content: Quote from: NiHaoMike on December 17, 2020, 01:25:45 pmBut this is the original design... full schematics in attach, it's from a Pioneer SX-990. There are some variant modules but the differences in these amp parts are not relevant. The signal path is basically (there are multiple input selectors and other passive-only filters) between these blocks:HEAD-AMP(RIAA) -> BalancePot->VolumePot -> CONTROL-AMP(HI/LO eq) -> CONTROL(passive filter) -> MAIN-AMP(PWR)When HEAD-AMP not used (other signal sources selected such as AUX and FM radio), the signal enters after the HEAD-AMP.The crackle noise is generated at the HEAD-AMP, because if we put the signal on AUX (enters after the HEAD-AMP) there's no crackle.The volume pot noise (no noise only at max volume) is there all the time.The only ICs in this device are in the FM radio and STEREO decoder, the rest is all discreet (even the FM front end).