I can't get a full sound out of any BJT amp I make - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 06, 2022, 03:38:45 pm
content: I don't see the problem, I've analyzed countless of my favorite (in terms of a full sound) amplifiers, tape players, etc. and apart for them using germanium pnp's instead of silicon npns which I'm using, it seems pretty much the same, but something's missing, it always just sounds plain and thin.when I'm used to making tube amps which more often have the opposite problem - too saturated sound.take for example this mic preamp I made:[ Attachment Invalid Or Does Not Exist ]it couldn't sound more harshly-thin if I tried. even on the low gain, with the NFB pot cranked until gain is like near unity, it still is thin and plain sounding.while a similar topology but with tubes sounds astonishing for just crappy cheap electrets used: (only major difference is that the tube amp I put a CF after the first mic stage, because I have that tube in the mic shell and thus I want a low impedance signal going over the wire, but a CF is just a CF)[ Attachment Invalid Or Does Not Exist ]what can I do to make the BJT preamp sound fuller, and just good in general, as does the tube preamp?(I do realize that triodes add 2nd harmonic distortion that adds 'warmth' but the difference is far too much to think it's just the minuscule amount of distortion making the tube one sound way better. there must be some big no-no in the bjt stages...)

author: Benta
author_rank: Super Contributor
timestamp: July 06, 2022, 06:29:59 pm
content: The problem is, that you're trying to design a solid-state amplifier the same way that you'd design a tube amp, and that just doesn't work.You can design a triode amp biasing it to the centre of linear operation, and you'll have a nice AC amplification of 7...8x (or something like that).Transistors don't work that way. They have immense hFEspread (perhaps 1:10 unless selected, where 1:3 can be expected) and are inherently non-linear.The design with transistors is done having lots of voltage (or current) gain and using feedback to linearize the response. Either local or global feedback, or a mixture.

author: fourfathom
author_rank: Super Contributor
timestamp: July 06, 2022, 06:57:47 pm
content: Well, this design does have some negative feedback in the first stage, and then the following amplifier and emitter-follower stages will be somewhat linear. But the middle transistor used to provide out-of-phase drive to the output stage is unbalanced by the 580 Ohm resistor in the DC feedback circuit and output stage biasing will likely be unbalanced without matched transistors. Both of these will cause significant distortion.As for "thin and plain-sounding", I wouldn't care to speculate.

author: IanB
author_rank: Super Contributor
timestamp: July 06, 2022, 07:17:50 pm
content: Quote from: Benta on July 06, 2022, 06:29:59 pmI believe also that the closest solid state approximation to a vacuum tube is an FET, and that similar circuit designs work for both? Wouldn't that be a better choice in a microphone preamp than BJT?

author: bd139
author_rank: Super Contributor
timestamp: July 06, 2022, 07:21:39 pm
content: Bin it all and use a low noise opamp. That’s the correct solution

author: Benta
author_rank: Super Contributor
timestamp: July 06, 2022, 07:27:01 pm
content: Quote from: IanB on July 06, 2022, 07:17:50 pmYes, that's true.But the parameter spread on FETs is immense, whereas it's probably +/-10% or less on a vacuum tube.

author: wizard69
author_rank: Super Contributor
timestamp: July 07, 2022, 05:35:22 am
content: I have no idea what a “full” sound means to you, I was under the impression the goal is to produce sound similar to a live performance.Given that I’d experiment with your circuitry to see if you can tune components better. If that gets you no where, consider the suggestion of others here and try out different architectures. Given those ideas do consider that many amps have gone bad due to caps drifting out of spec.

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 06:55:53 am
content: Are your 2SC.. "PNP" transistors at the output wired properly??

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 07:41:42 am
content: With 560ohm (your value above) in the emitter of the second NPN the gain of entire amplifier is hardly 1 in my sim, with 2k2 for example the gain will be 80..BTW., 2SC1267 is an UHF transistor, afaik..PS: Also you have to play a little bit with the values of the first and second resistor in the emitter of the second NPN transistor in order to get an equal amplitude in both channels (if required)..

author: mikerj
author_rank: Super Contributor
timestamp: July 07, 2022, 09:51:03 am
content: Quote from: ELS122 on July 07, 2022, 06:57:14 amIt doesn'tlookcorrect, though it's somewhat dependant what gets plugged into the three pin connector. Your PNP followers can only sink current, but they have their collectors tied to the positive rail so the emitter load (which would need to be supplied by the external circuit) would need to be tied to an even higher voltage. This also means the biasing would be highly dependant on the external voltage. The output isn't capacitively coupled so common mode voltage will be way above 0v. Either way it's not equivalent to the valve circuit.

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 10:14:06 am
content: Quote from: mikerj on July 07, 2022, 09:51:03 am..it would work however, provided the three pin connector is wired as in my simulation above (my assumption is - it is made so)..

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 12:14:50 pm
content: OK, I get gain=228 with your schematics values at the 600ohm load. My signal starts at the capsule, however (out of curiosity I used 3 soviet transistors there)..In my above simulation I had 6.2V zener instead of 18V one in your schematics, grrh.The output PNPs work (BC640 there). I would use some which work fine with those 48V phantom voltages.Your KT315 used in simulation.Are those KT315 such small orange boxes?

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 12:50:39 pm
content: This is without the capsule, amplifier only, Rsig_gen=0ohm..Gain=566 with 1mVpp input, but it limits with input at 10mVpp.So be careful with the input levels..

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: July 07, 2022, 03:07:07 pm
content: Valve amps with little or no feedback have second harmonics due to none linearity.Transistor amps that are flat frequency response will sound plain compared to a valve amp.Sound level is important too as the ear frequency response drops off at high and low frequencies.The biggest distortion in an amp system is the speakers which are far from flat.Just look at your speakers pdf.

author: fourfathom
author_rank: Super Contributor
timestamp: July 07, 2022, 04:08:32 pm
content: Quote from: imo on July 07, 2022, 12:50:39 pmIn this circuit, the AC gain is primarily set by the ratio of R5 (the collector-base feedback resistor, 470K) and the output impedance of the source. In the simulation the source has a zero output Z, so the gain is instead limited by transistor HFE(and other parameters.) The output Z of a typical(?) microphone will certainly higher.Also, I believe that the "phantom" 48V supply is limited to about 15mA, so perhaps those resistors should be about 3.3K? You would still need an AC-coupled 600 Ohm load. This may make little difference though.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 07, 2022, 04:22:13 pm
content: Quote from: Benta on July 06, 2022, 06:29:59 pmThat needs some more emphasis.I was wondering why those last two transistors were connected in such a completely bonkers (and not working) way.I don't know much about those obsolete glass botles, but for silicon it does not work that way.

author: fourfathom
author_rank: Super Contributor
timestamp: July 07, 2022, 04:37:43 pm
content: Quote from: Doctorandus_P on July 07, 2022, 04:22:13 pmThat was also my first impression, but the output stage actually works when you feed it from a 48V phantom-power source. What at first looks like what you would see with a tube-based class-AB transformer output stage is actually two continuously-conducting emitter followers. The single base-bias resistors don't establish much of a VCEoverhead (and the amount is poorly controlled) so it's not going to provide much available signal-swing, but it does work. Adding resistors from base to emitter might better-establish the VCEand improve the dynamic range.

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 04:55:01 pm
content: Quote from: fourfathom on July 07, 2022, 04:08:32 pmIt depends, no idea what OP has wired at the other side of the mic connector..Rg now 5k, it creates a divider with Zin so that dependency is clear..AC coupled 600ohm load, here it is with 10mA via PNP Ie current.The input amplitude steps from 1mVpp (green) up to 10mVpp.All important voltages and currents depicted for clarity.Btw, I am not designing this mic preamplifier. Just helping OP to find the issue why it sounds "not good".PS: I've been using oxygen free copper wires in this simulation, thus it should sound much better now..

author: fourfathom
author_rank: Super Contributor
timestamp: July 07, 2022, 05:40:54 pm
content: Quote from: imo on July 07, 2022, 04:55:01 pmBut are the copper molecules atomically aligned? Perhaps that's why the thin sound?But honestly, where are we on frequency response and distortion? The small-signal distortion looks adequate, but that will depend a lot on the microphone impedance (due to the impedance-dependent negative feedback canceling the raw Q1 distortion). And apparently the gain-difference in the + and - outputs doesn't matter because of the inversion/summing in the following amplifier.When I hear the description "thin and plain sounding" I think "not enough bass-boost and no compression or soft peak-clipping", rather than there being a distortion problem. Perhaps the problem is that the preamp design is just too clean and flat?

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 08:43:48 pm
content: The Zin of the preamp is aprox 470ohm (1k to 100kHz), therefore below the TH Distortion (at 1kHz, 5kHz, 10kHz, 20kHz) with Rg=470ohm and input signal amplitudes 0.1mVrms, 1mVrms and 10mVrms.Code:################################Vin = 0.1mVrms################################.step freq=1000N-Period=10Fourier components of V(out1)DC component:1.28776e-005HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 11.000e+037.966e-021.000e+0018.74°0.00°22.000e+039.655e-051.212e-03-42.82°-61.56°33.000e+031.346e-071.690e-0647.41°28.68°44.000e+032.262e-082.840e-07-2.82°-21.56°55.000e+031.701e-082.136e-07-0.92°-19.66°66.000e+031.408e-081.767e-07-0.79°-19.53°77.000e+031.216e-081.526e-07-0.57°-19.31°Total Harmonic Distortion: 0.121199%(0.121208%).step freq=5000N-Period=10Fourier components of V(out1)DC component:-0.000138327HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 15.000e+038.391e-021.000e+001.57°0.00°21.000e+041.093e-041.303e-03-81.90°-83.47°31.500e+046.819e-068.127e-05179.64°178.07°42.000e+045.228e-066.230e-05-179.09°-180.66°52.500e+044.179e-064.981e-05-179.26°-180.83°63.000e+043.484e-064.152e-05-179.41°-180.98°73.500e+042.985e-063.557e-05-179.47°-181.04°Total Harmonic Distortion: 0.130923%(0.310049%).step freq=10000N-Period=10Fourier components of V(out1)DC component:-0.000382476HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 11.000e+048.383e-021.000e+00-2.81°0.00°22.000e+041.159e-041.382e-03-85.12°-82.31°33.000e+048.799e-061.050e-04178.38°181.19°44.000e+046.779e-068.087e-05-179.10°-176.29°55.000e+045.472e-066.528e-05-179.98°-177.17°66.000e+044.489e-065.355e-05-179.09°-176.28°77.000e+043.924e-064.680e-05179.92°182.73°Total Harmonic Distortion: 0.139217%(0.424558%).step freq=20000N-Period=10Fourier components of V(out1)DC component:-0.000477509HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 12.000e+048.304e-021.000e+00-8.54°0.00°24.000e+041.366e-041.644e-03-83.57°-75.03°36.000e+045.397e-066.499e-05171.34°179.87°48.000e+043.594e-064.328e-05-168.47°-159.93°51.000e+053.989e-064.804e-05174.32°182.86°61.200e+051.941e-062.337e-05-167.80°-159.27°71.400e+053.116e-063.752e-05177.43°185.97°Total Harmonic Distortion: 0.164761%(0.329465%)################################Vin = 1mVrms################################.step freq=1000N-Period=10Fourier components of V(out1)DC component:0.00102319HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 11.000e+037.959e-011.000e+0018.73°0.00°22.000e+039.604e-031.207e-02-42.75°-61.47°33.000e+031.226e-041.541e-0458.49°39.77°44.000e+031.759e-052.210e-05-29.65°-48.38°55.000e+033.083e-063.873e-06-18.10°-36.83°66.000e+031.311e-061.648e-067.11°-11.61°77.000e+031.158e-061.455e-06-0.83°-19.55°Total Harmonic Distortion: 1.206769%(1.206845%).step freq=5000N-Period=10Fourier components of V(out1)DC component:5.46281e-005HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 15.000e+038.382e-011.000e+001.55°0.00°21.000e+041.104e-021.317e-02-76.53°-78.08°31.500e+042.314e-042.760e-0466.41°64.86°42.000e+048.082e-059.642e-05-170.69°-172.24°52.500e+043.904e-054.657e-05-169.81°-171.36°63.000e+042.841e-053.390e-05-179.09°-180.64°73.500e+042.527e-053.015e-05179.65°178.10°Total Harmonic Distortion: 1.317053%(1.338516%).step freq=10000N-Period=10Fourier components of V(out1)DC component:-0.00138848HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 11.000e+048.374e-011.000e+00-2.84°0.00°22.000e+041.176e-021.405e-02-78.18°-75.34°33.000e+044.238e-045.061e-0452.09°54.94°44.000e+041.092e-041.304e-04164.70°167.55°55.000e+044.997e-055.967e-05-172.64°-169.80°66.000e+043.117e-053.722e-05-176.45°-173.61°77.000e+042.691e-053.213e-05179.82°182.67°Total Harmonic Distortion: 1.405555%(1.433105%).step freq=20000N-Period=10Fourier components of V(out1)DC component:-0.000464281HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 12.000e+048.295e-011.000e+00-8.60°0.00°24.000e+041.385e-021.670e-02-79.50°-70.90°36.000e+047.729e-049.318e-0419.04°27.64°48.000e+041.002e-041.209e-04106.10°114.70°51.000e+052.972e-053.584e-05163.41°172.00°61.200e+051.278e-051.541e-05-173.48°-164.89°71.400e+059.163e-061.105e-05-177.92°-169.32°Total Harmonic Distortion: 1.672311%(1.675141%)################################Vin = 10mVrms################################.step freq=1000N-Period=10Fourier components of V(out1)DC component:0.524173HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 11.000e+033.659e+001.000e+0013.14°0.00°22.000e+031.296e+003.541e-01-63.39°-76.53°33.000e+034.234e-011.157e-0136.55°23.41°44.000e+036.234e-011.704e-01-37.31°-50.46°55.000e+031.166e-013.187e-02-100.92°-114.06°66.000e+031.401e-013.829e-02-16.56°-29.71°77.000e+034.968e-021.358e-02-68.46°-81.61°Total Harmonic Distortion: 41.291267%(41.427370%).step freq=5000N-Period=10Fourier components of V(out1)DC component:0.692396HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 15.000e+033.680e+001.000e+000.47°0.00°21.000e+041.177e+003.197e-01-88.03°-88.51°31.500e+045.540e-011.505e-01-0.88°-1.35°42.000e+046.413e-011.743e-01-87.12°-87.59°52.500e+045.721e-021.554e-02-142.03°-142.50°63.000e+041.954e-015.310e-02-90.38°-90.85°73.500e+046.391e-021.737e-02-159.15°-159.62°Total Harmonic Distortion: 39.826095%(39.981419%).step freq=10000N-Period=10Fourier components of V(out1)DC component:0.779796HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 11.000e+043.725e+001.000e+00-3.16°0.00°22.000e+041.067e+002.865e-01-94.13°-90.96°33.000e+046.601e-011.772e-01-12.69°-9.52°44.000e+046.230e-011.673e-01-100.11°-96.95°55.000e+046.892e-021.850e-02-79.46°-76.29°66.000e+042.263e-016.076e-02-112.20°-109.03°77.000e+046.197e-021.664e-02-159.91°-156.75°Total Harmonic Distortion: 38.181661%(38.413569%).step freq=20000N-Period=10Fourier components of V(out1)DC component:0.85151HarmonicFrequencyFourierNormalizedPhaseNormalizedNumberComponentComponentPhase 12.000e+043.758e+001.000e+00-8.50°0.00°24.000e+049.633e-012.563e-01-102.24°-93.75°36.000e+047.444e-011.981e-01-30.50°-22.00°48.000e+045.851e-011.557e-01-117.97°-109.47°51.000e+051.586e-014.221e-02-90.49°-81.99°61.200e+052.395e-016.373e-02-144.36°-135.86°71.400e+059.036e-022.404e-02-159.18°-150.68°Total Harmonic Distortion: 36.820333%(37.109745%)

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2022, 09:00:17 pm
content: .. and the freq response below.PS: From the frequency response it seems it lacks bass, I would double or triple the values of the coupling capacitors to move it somewhere down to 30Hz. The high side ends at 79kHz, it seems to me it is set too high (the large the BW the more noise then).The THD is perhaps on the higher side for signals around 1mVrms input (1.3%, but I will definitely not notice that) and around 10mVrms input it is 40% - that indicates an issue, imho.The electret capsule will certainly produce signals higher than 1mVrms..