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Messages - Loudthud

#286
Amplifier Discussion / Re: Randall RG80/100ES QUESTION
October 01, 2012, 08:05:10 PM
I'm not familiar with the head. Can you post some pictures? Any numbers on the connectors?
#287
Quote from: JayFett on September 24, 2012, 09:51:55 PM
Thanks for the answers.
So... if the power supply caps are intentionally "under-sized", will the amp then exhibit voltage sag, and mimic the 'note bloom' that certain tube amps have when played at higher volume? Or will it just show up as weak bass in the chip guitar amp? Or just noise?

I've been looking at this issue in an attempt to mimic the dynamics of tube guitar amps. Note bloom seems to be a function of gain change with power supply sag and recovery. Hard to do with solid state amps because they use so much feedback unless a deliberate compressor type circuit is used. Note attack is a function of power supply sag but the power amp has to swing to the rails.

There is a certain amount of power supply ripple that gets to the speaker when a tube power amp clips. I'm using a TDA2030 and can run off of an AC wall wart or a regulated DC power supply. The difference is subtle, but I would say the tone is a little more grunty when the AC supply is used. With my current proto I only get ripple on one rail, I'll have to go to a bridge setup to get it on both sides of the speaker waveform.

Sag also affects the preamp in tube amps. Most SS amps regulate the supply(s) to the preamp and with opamps there wouldn't be any gain change anyway. My amp is using a single rail with a capacitor coupled speaker and an unregulated supply to the preamp. The output bounces around a little and I like the sound. I'm trying to get something put together so other people can play it to see if they like it.
#288
Preamps and Effects / Re: Look Ma, NO POTS!
September 28, 2012, 12:39:02 PM
I did some experiments with a matched pair of J201s. (Guessing Idss within 2%, Vgoff within 5%). I used a 1.5K Source resistor on one side and the servo circuit on the other. 33uF capacitors bypassed the Sources. To be fair, I used a dual 50K pot to adjust the Drain resistor of both JFETs. That way when the pot was adjusted to give equal Drain voltage at DC, both JFETs would have very close to the same gain. The generator was connected directly to the Gates, no stopper and no coupling cap (important). Signals with Hi/Lo times of 40/60, 60/40, 20/80 and 80/20 at amplitudes of 200mV and 600mV were applied at 1KHz. At 100Hz you could start to see a little tilting on the servo side Drain, maybe the time constant needs to be longer,

Source voltage was measured for each duty cycle at supply voltages of 9V, 20V and 30V. In the vast majority of cases the shift in Source voltage taken as a percentage of change from the DC value was something less than twice as much in the servo circuit. I plan to retest with a coupling cap between the generator and the Gates. This should make the integrator see a consistant DC Source voltage and only shift bias when the signal clips.

In the interest of full disclosure, changes had to be made in the circuit at 30V. 33K was added in series with both sides of the Drain pot and 680 ohms added from Source to ground on the servo side because the opamp couldn't swing low enough with the 1K between it's output and the Source.

Opamp selection: A TL072 could be used but they don't like the output to swing very close to the rail. That could be fixed with a resistor from the Source to ground. A JFET input opamp would allow the integrator resistor to be increased. I'm sure better parts are available.
#289
Preamps and Effects / Re: Look Ma, NO POTS!
September 27, 2012, 03:14:31 PM
Tip: If you want to set all the JFETs in your preamp stages to the same drain voltage, you can use the same voltage divider (R1/R2) for all the opamps.

Cathode bias will shift in a tube amp if it is clipping non-symmetrically or the operating point is off center. I will experiment later today driving two JFET stages at the same time. One with servo bias and one with ordinary source bias and compare bias shifts (source voltage).

To make an input stage "pedal friendly", I try to duplicate what the output clipping of the first stage looks like in reference to the input. With a 30V preamp supply rail, it looks like the gain needs to be less than 10. I use diode clipping on the input because gate current causes the output to clip too sharply and gate current doesn't always start when the output clips.
#290
Preamps and Effects / Re: Look Ma, NO POTS!
September 26, 2012, 11:59:15 AM
Quote from: KMG on September 26, 2012, 11:30:58 AM
Adding servo you'll loose most benefits of jfet stage. In this case it'll sound like simple opamp with clipping diodes.
Simpler tunung but poorer sound.

Please list those benefits and note which ones you think will be lost. Then build the circuit to confirm.
#291
Preamps and Effects / Re: Look Ma, NO POTS!
September 26, 2012, 11:18:36 AM
Quote from: KMG on September 26, 2012, 02:56:32 AM
Accurate emulation of grid current (shift of operatin point) often means more than a shape of clipping.
Look how tube stage respond on "burst" input signal.

This thread isn't about tube emulation. In fact, I left those components off the schematic. It's about JFET preamps for novice builders who don't even own a DVM or know how to measure anything. If you want to over analyse things, tell me why the opamp doesn't oscillate or if it will with changes to time constants in the loop.  :)  :)  :)
#292
Preamps and Effects / Look Ma, NO POTS!
September 25, 2012, 10:10:37 PM
I ran across this circuit on an old Sunn schematic. It's a servo to set the operating point of a JFET so you don't need a pot. Resistors R1 and R2 form a voltage divider, the voltage at the center will be the Drain voltage. The voltage divider will track changes in supply voltage and the opamp will adjust the Source voltage to bring the Drain voltage back.

R3 needs to be low enough that the opamp can still supply the Drain current and not have to swing it's output too close to ground. If the JFET needs a source voltage too close to ground, add a resistor across the 47uF cap, lower than needed to bias the JFET and let the opamp pull it positive to reduce Source current. If you want to add a treble peaking R/C or Fetzer resistor in the Source, add it between the 47uF cap and the Source.

I built this circuit on a proto board with a J201. When I plugged in a MPF102 the bias adjusted itself and the gain was very close to where it was with the J201. The bias adjusts perfectly changing the supply from 9 to 32V. The bias can drift when the output is clipping. You'll get a little jump in bias when the signal cuts off. A slight adjustment to the R1/R2 ratio will fix this.
#293
Schematics and Layouts / Re: Randall RG100 hybrid
September 20, 2012, 11:47:01 AM
Randall's power amp is not what you would call "mixed mode" feedback, it's current only feedback with DC correction. C34 blocks any AC voltage feedback, only allowing DC and very low frequencys to pass. The current feedback signal from R63 gets divided down by R52 and R44. C36 and C33 block any DC feedback through that path that would otherwise kill the DC coming back through R45.

This same type arrangement could be used with a Chip or other power amp if desired, although stability might become an issue. C32 is a fairly high value limiting the high frequency open loop gain of the Randall amp.
#294
Schematics and Layouts / Re: Randall RG100 hybrid
September 18, 2012, 01:04:23 PM
Break the connection of C1 to ground and connect it to the top of R12. If you want to experiment, connect a 25 ohm wirewound pot (1 or 2W) across R12 and connect C1 to the wiper. That way you can adjust between voltage mode and mixed mode feedback. There will be some gain change as you adjust the pot. R12 can be 0.22 5W (or higher). JM basically gave you the answer in post #21.
#295
I don't really like Veroboards, I've never used one. That track that goes to Drive 2 goes half way across the board through the output section making it a good path for stray pickup and oscillation. If it's not too much trouble, cut that track around column 15 and attach the Drive 2 wire there.
#296
I would first try removing C6 (layout numbers). If the sound is too buzzy, try installing C6 from gate to ground across R11.
#297
C9 orientation depends on what the following stage looks like. It's correct for the circuit you took it from because the following circuit has a positive voltage on it. Circuits with JFETs need to be tweeked to run properly, usually because you can't get the exact parts the designer used. This circuit would run better on 24V to 30V. The circuits at Runoffgroove make better DIY projects because they include trim pots and instructions needed to get them working without a lot of messing around. You will need a DVM to measure voltages.
#298
Schematics and Layouts / Re: Randall RG100 hybrid
April 04, 2012, 08:27:01 PM
Quote from: allan_belt on April 04, 2012, 12:15:14 AM
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???

Those FETs are pretty close. The transconductance of the 2N5457 is spec'd at 1000 to 5000 uMho. The 2N5484 is spec'd at 3000 to 6000 uMho so on average, it's a little higher but it depends on the exact part you are using.

Quote from: allan_belt on April 04, 2012, 12:15:14 AM
2. tested the sound out of the preamp section into another guitar amp and the sound is great!!! punchy and Heavy  :dbtu: but when i connected to the power amp section presented here the sound is weak and too trebly for my taste, is it normal??

I would look for a bad component or wiring error. Try feeding another preamp into the power amp. Or just feed your guitar straight into the power amp.
#299
You might try just about any dual opamp. Install a socket so it is easy to swap parts. The 4558 is popular and of course the TL072 or 5532. Report back on how they sound and if the fizz goes away.
#300
Amplifier Discussion / Bandwidth of Power Amp
March 15, 2012, 06:27:45 PM
I'm prototyping a solid state power amp. It's class A, sort of a JLH meets Nelson Pass thing. It runs without any compensation although there is a slight oscillation. I added a couple of caps and that killed the oscillation and cleaned up the square wave response. The -3dB bandwidth is still over 500kHz. So I put the typical RC on the input, knocked the bandwidth down to 40kHz. I could go lower, just change one capacitor. I like the single PNP transistor input borrowed from JLH. When you overdrive it, you get a 40/60 square wave.

My question is, how much bandwidth should a power amp have? I know it's important that when you are overdriving it, the slopes should get steeper the harder you drive it. The amp sounds ok to me so far, even without the bandwidth limit.