Allan Posted November 4 Share Posted November 4 (edited) Anyone done it, by adjusting amp bias? Amp gain should drop proportionally with bias voltage, right? Edited November 4 by Allan Link to comment Share on other sites More sharing options...
muon* Posted November 4 Share Posted November 4 Changing the bias changes the operating point of the amp, if I'm not mistaken that will change the frequency response of the amp. Link to comment Share on other sites More sharing options...
Allan Posted November 4 Author Share Posted November 4 49 minutes ago, muon* said: if I'm not mistaken that will change the frequency response of the amp. exactly what I'm hoping for, I want more emphasis from the high gain pre Link to comment Share on other sites More sharing options...
muon* Posted November 4 Share Posted November 4 (edited) 1 minute ago, Allan said: exactly what I'm hoping for, I want more emphasis from the high gain pre Lower bias will likely emphasise the low end. Edit: That's what I got from lower bias settings with a Sansui AU-555. Edited November 4 by muon* 1 Link to comment Share on other sites More sharing options...
Allan Posted November 4 Author Share Posted November 4 (edited) 6 minutes ago, muon* said: Lower bias will likely emphasise the low end. Edit: That's what I got from lower bias settings with a Sansui AU-555. If I could I'd buy the coffees Edited November 4 by Allan Link to comment Share on other sites More sharing options...
aussievintage Posted November 4 Share Posted November 4 13 hours ago, Allan said: Amp gain should drop proportionally with bias voltage, right? Without knowing the topology or having a schematic, how can you tell ? I would venture that, in most designs, the answer is no. In most amps. gain is determined by the feedback loop. Link to comment Share on other sites More sharing options...
Allan Posted November 5 Author Share Posted November 5 Feedback contributes to gain Link to comment Share on other sites More sharing options...
aussievintage Posted November 5 Share Posted November 5 (edited) 2 hours ago, Allan said: Feedback contributes to gain No, it determines the gain, as something less than the available open loop gain. Increasing or decreasing the open loop gain will not change the gain of an amplifier that has negative feedback setting the actual gain. Edited November 5 by aussievintage Link to comment Share on other sites More sharing options...
muon* Posted November 5 Share Posted November 5 (edited) Does depend on topology, too much GNFB in some designs can reduce max power output, can also cause instability. Depends. Edited November 5 by muon* Link to comment Share on other sites More sharing options...
Ihearmusic Posted November 5 Share Posted November 5 Dropping the bias will put the operating point of the tube into the non linear region and therefor increase your distortion. If you want to drop your gain you have to reduce the B+ to whatever tube you are aiming to reduce the gain and then recalculate the proper bias point. But really it also depends a lot on the design. If it is a multi stage design one should rework all the parameters for each stage so not to have one stage run out of headroom and so on and on. If you want optimal performance you have to put in the work........... 3 Link to comment Share on other sites More sharing options...
Addicted to music Posted November 5 Share Posted November 5 23 hours ago, Allan said: Anyone done it, by adjusting amp bias? Amp gain should drop proportionally with bias voltage, right? In reference to solid state, changing the bias does not change gain. What it does is to turn on the transistors or output devices so as it cross over the zero point its linear and not non-linear. Feedback determined gain. Link to comment Share on other sites More sharing options...
muon* Posted November 5 Share Posted November 5 @Allan can you tell us what the amplifier is, please. Link to comment Share on other sites More sharing options...
Allan Posted November 5 Author Share Posted November 5 Mark Levinson 27 Link to comment Share on other sites More sharing options...
muon* Posted November 6 Share Posted November 6 I wouldn't change how the Levinson operates, I'd just try and use a pre that matches it. Link to comment Share on other sites More sharing options...
Allan Posted November 6 Author Share Posted November 6 (edited) Why wouldn't you change its bias, so long as it's not a significant change and things don't overheat there's absolutely no issue. Go read up on this at DiyAudio If fact Nelson Pass says you can bias by heat.. not that I would but I do monitor it and stay well within spec for the amp in question. In fact when I bought the ML its bias was a country mile low, probably why it was sold as it sounded like **** when I got it home. I biased it up properly and the thing suddenly sang like a crazed blackbird on heat! I've rebiased my F5 many a time, changed its gain and also removed the current limiters, all made a notable change in sonics... not to mention P3. In fact I rebias it 2 times a year, before and after summer. Low in summer and crank the **** out of it for winter... as do many other people who've built one Edited November 6 by Allan Link to comment Share on other sites More sharing options...
aussievintage Posted November 6 Share Posted November 6 1 hour ago, Allan said: Why wouldn't you change its bias, because you still seem, erroneously, to believe bias will change the gain (proportionally). It won't help you. 1 Link to comment Share on other sites More sharing options...
Addicted to music Posted November 6 Share Posted November 6 7 hours ago, Allan said: Why wouldn't you change its bias, so long as it's not a significant change and things don't overheat there's absolutely no issue. Go read up on this at DiyAudio I dont go there for many reasons 7 hours ago, Allan said: If fact Nelson Pass says you can bias by heat.. not that I would but I do monitor it and stay well within spec for the amp in question. I doubt that NP would be caught saying that, unless he has graphed out voltage vs temp and both are measured 7 hours ago, Allan said: Why wouldn't you change its bias, You follow what the specs tell you what to do. Changing the bias outside the range of whats documented does other nasties to the overall performance, even if you think your ears tell youn its an improvement when its not 2 Link to comment Share on other sites More sharing options...
Allan Posted November 7 Author Share Posted November 7 when initially biasing an amp from being built, you start at zero and work it up to spec, never the opposite... if it oscillates before quiescent is acheived there's a problem. Link to comment Share on other sites More sharing options...
MattyW Posted November 13 Share Posted November 13 On 06/11/2024 at 2:35 PM, Allan said: If fact Nelson Pass says you can bias by heat.. not that I would but I do monitor it and stay well within spec for the amp in question. I know not really relevant to your thread topic, though my Absolute Audio Labs SIT-A15 SE is biased by heat..... And damn if it doesn't sound glorious in the middle of summer when it's stinking hot. 1 Link to comment Share on other sites More sharing options...
georgehifi Posted November 20 Share Posted November 20 @Allan Amount of global feedback on a tube amp determines gain/distortion percentage/stability/output impedance (damping factor/bass control) same as it does on solid state amps. Amount of bias determines the amount of Class-A operation you have before it goes into Class=B, it does not really effect gain unless somethings wrong, if you have too much bias the amp will burn up just at idle. (Nelson Pass had a good rule of thumb to follow on solid state biasing, hot day after 2 hrs at idle palm of hand on heat sink should not be painfully uncomfortable.) Cheers George 1 Link to comment Share on other sites More sharing options...
aussievintage Posted November 20 Share Posted November 20 55 minutes ago, georgehifi said: Nelson Pass had a good rule of thumb to follow on solid state biasing, hot day after 2 hrs at idle palm of hand on heat sink should not be painfully uncomfortable. Also known as the hot coffee cup rule. If you want to push it then, if you wet your fingertip with saliva and touch a component, it should NOT go sszzzt Link to comment Share on other sites More sharing options...
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