Menu

Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - Loudthud

#226
I have found that the more runcible a circuit, the more it sounds like a tube amp.

In the Poptronics circuit, the AC gain is not affected by the bias Voltage set pot R5 because that pot is bypassed by capacitor C3. One little quirk of the circuit is that setting R5 doesn't track the main power supply. As the power supply sags, the Voltage at the + side of C5 doesn't move much. This works to your advantage if you use a soft power supply. You get more compression and touch sensitivity.

I have attached my version of a similar circuit. No global AC feedback puts the gain up around 100. The output impedance is not as low as you might think, but it depends on the beta of the output transistors you use. In the one I built, it was around 5 Ohms. Power the preamp from a decoupled supply from the main Voltage rail. Add enough capacitance to stop the low frequency motorboating.
#227
Check for a broken solder joint on one of the filter capacitors.
#228
I would siggest that the cap go to ground so that it reduces the feedback from both Current and Voltage sources. Also there needs to be a limit to the reduction in feedback. Perhaps a 47 Ohm resistor in series with the cap. The best way to look at this is with simulation. Less smoke that way  8)
#229
The 38K in TSC is a rough approximation of the output impedance of a 12AX7 with a 100K plate resistor.
#230
Preamps and Effects / Re: The JFET Bender Preamp
April 22, 2015, 05:26:01 PM
I thought I would provide a little background on how the JFET Bender emulates a triode. Some of this has been covered before in this thread: http://www.ssguitar.com/index.php?topic=2381

The first scope shot is an X-Y display of a Chinese 12AX7 with a 100K plate load to a 300V supply. Input is the horizontal axis, the plate is the vertical axis. As the input moves negative (to the left) there is a reduction in gain. This is where the JFET needs to become a higher impedance to reduce the gain. The next photo shows the JFET Bender. To make a clearer display on the scope, the 1uF capacitor in series with the feedback resistor is shorted out. To better match the 12AX7, the input clamp was reduced to three diodes as shown in the schematic. The last scope photo shows what happends when the bias is reduced on the JFET taking it out of it's nonlinear region. The input clamp is still working on the right side of the input.
#231
Preamps and Effects / The JFET Bender Preamp
April 17, 2015, 05:54:14 PM
This thread is not about a distortion pedal called the Tone Bender.
It is about a preamp that emulates a Blackface Fender tube preamp.

In a previous thread I explored ways of getting opamps to act more like a 12AX7. Link: http://www.ssguitar.com/index.php?topic=2381 This is a continuation of the same idea except a JFET is used in the feedback network. A JFET is ideal for this circuit because it doesn't have a dead zone where it stops conducting current if the bias is set right. I hope some of you will try this idea in some exsisting designs to add some low level even order distortion. I call it the JFET Bender because it bends the straight line input-output relationship an opamp normally produces.

The JFET is used as a variable resistor in a region where it's resistance changes at different current levels. MPF102 JFETs were used because they had a bendable resistance in the right range. I originally tried 2N4391-3 but the pot setting was way too touchy. I had to install multi turn pots. The MPF102s have an Idss usually between 8 and 12mA. The Curve Tracer shot shows how Voltage and Current behave around zero volts. When the Voltage swings negative, the curve is fairly linear but as it swings positive, the JFET starts to go into constant current mode and the current levels out. Adjusting the Gate Voltage lets you adjust the amount of bend and thus the amount of distortion.

The distortion generated is mostly pure 2nd order at low output levels. Some 3rd order creeps in as the output of the stage reaches 10V pk-pk. The distortion rises to about 10% (this is adjustable via the pot) when the output of the opamp starts to clip. Because the gain is not the same on plus and minus halves of the waveform, one side clips before the other. To adjust the pot, set the pot near zero Volts. That's where maximum gain is. Apply a signal and adjust the pot until the gain is reduced to about 80% (-2dB) the max value. Or just adjust it by ear for a little gain reduction. If the pot is set too far negative, the JFET cuts off and the gain falls to 1. You can find a place where there is severe clipping on one side of the waveform if desired.

For those paying attention, the gain reduction caused by an N channel JFET in the first experiment is backwards compared to what it's like in the first stage of a tube guitar amp. What is really needed is max gain on positive inputs (closer to vertical line on the curve tracer) and a slow reduction in gain for negative inputs (the curve of the JFET lays over horizontally). I played guitar through a couple of stages setup backwards like this with a tone stack in between and it didn't sound right. It just didn't have the Fendery clean tone I was looking for.

Next I tried a J201 connected differently to invert the distortion. The Drain is grounded. When the Gate and Source swing positive, you have the bulk resistance Rds(on) in parallel with the Gate to channel diode. This is fairly linear until the diode starts to conduct. When the Gate and Source are pulled negative, you get the Idss curve and it lays over (resistance increases) in the third quadrant. The disadvantage of this circuit is that there is no adjustment and the resistors in the feedback network have to be adjusted to get the current and voltage in the right range for the J201. This circuit sounded better as a first stage but there was not enough distortion at the gain I wanted for the first stage. Distortion was in the range of 1% to 4% at 1Vrms input depending on the impedance in the feedback network. Too big a resistance in series with the JFET in the shunt arm of the feedback network limits the amount of distortion the JFET can generate.

Lastly I looked for a P-channel JFET that had favorable characteristics. J174 and J175 were nowhere close. The 2N5462 looked good. It is a near compliment to the MPF102. This worked in the original circuit, all that was needed was to tweek the pot for positive bias on the Gate. A 2N4343 was also used, it worked about the same.

Since a tube preamp has mostly inverting stages, a non-inverting opamp preamp will have to flip the distortion on alternate stages. I think not doing this is why the first try didn't sound very good. So a P-channel JFET is in the first stage and an N-channel JFET is used in the second stage. The second stage needs a gain around 50.

When the gain is high, there is not much Voltage available for the JFET to distort. On the second stage the feedback resistors set the gain at 100 and the pot is adjusted to get 6dB of gain reduction. This gets the distortion up to around 10% before the opamp starts to clip.

The preamp is configured to mimic the Fender Blackface preamp. The first stage has low gain to provide about the same headroom as a tube, the diodes on the input clip the incoming signal so as to be pedal friendly and protect the opamp. The diode on the second stage is inverted because the input stage opamp is non-inverting.

To test the circuit out, I used a Peavey Mark III Standard from the 80's. It has two opamp based preamp channels, one with a FMV tone stack, and a handy power amp input on the front panel. The power amp is a 260D (130W at 4 Ohms) with Voltage only feedback. Without the capacitors in series with the JFETs, there was too much subsonic feeding through to the power amp so the caps were added to limit subsonic response.The amp gives a very convincing Blackface tone for both bass and guitar. I especially like it for bass, but the caps could be increased slightly to give a little bit more low end.
#232
Amplifier Discussion / Re: Ampeg BT15
April 14, 2015, 06:46:07 PM
I had an amp very similar to this, I think it was a BT25 head. It didn't have the electroluminescent panel, but I remember the 300 Ohm resistors. Got rid of it in '83. It didn't have excessive hum. The Dynaco amps had a regulated power supply. They hum when the regulator pass transistor shorts out.

Peter Baxandall had a diode in some of his quasi-complimentary designs, but it was just in series with the PNP emitter and had a resistor across it. He claimed it improved thermal tracking. Some people started calling it the "Baxandall Diode".
#233
Amplifier Discussion / Re: Ampeg BT15
April 14, 2015, 02:38:27 PM
There was a similar biasing scheme in the Dynaco Stereo 120 with the 300 Ohm resistors forcing the PNP driver to conduct. That amp didn't have a Vbe multiplyer, the power amp had 6 transistors total. The circuit never became very popular as technology was changing fast at the time and better PNP transistors became available.

Notice that Ampeg used the wrong symbol for input JFETs, they are actually P-channel Source followers.

Check the diodes in the rectifier bridge. One may have died with the new filter caps.
#234
What's up with the circuitry around IC2B on page 2 of the power amp schematic? Looks like a graphic EQ except there are no pots, just fixed resistors setting the gain on all the bands. Then in the feedback network around the power amp, R30 and C26 looks like a resonance type bass boost. Seems like the factory is setting the EQ more than the player.
#235
Tubes and Hybrids / Re: PCB question
February 27, 2015, 07:27:31 PM
On small areas I use alcohol and a cotton swab (Q-Tip) but on large areas I use an old tooth brush and alcohol or a commercial cleaner such as Puretronics Flux Remover available in the US at Frys. It usually helpful to mop up after a good scrub with a paper tissue before the cleaner evaporates.

The flux cleaner mentioned contains Toluene, Heptane, Isopropyl Alcohol and 1,1 Difluoroethane.
#236
Quote from: J M Fahey on February 06, 2015, 01:42:53 AM
Well, that's a usual problem .... but in this particular case, you are being unfair with poor old Marshall (RIP)

In page 2 of the schematic, it's clearly shown that +/- 15V feeds pins 8 and 4 of all dual Op Amps (RC4558 and NE5532) and pins 13 and 4 of DG212.

It is also clearly shown that said DG212 gets +5V on pin 12 and that pin 5 is grounded.

So stop bashing poor old Jim  :trouble

:lmao:

The only reason that schematic was drawn is to generate a netlist so that a circuit board could be layed out. The person who does the layout probably never even looked at the schematic. The netlist utility will tell the circuit board program to connect everything that is defined as "+15V", but not to the output of the 7815 because it's not defined as going to the net "+15V". See how there is "+5v" near the output of the 7805? There is also a "+5V" in the upper left corner near R79. Those two points would get connected.

Poor old Jim probably wouldn't even recognize what his company has become. All over that schematic there is text on top of wires, components and other text. This tells me that there is no pride in workmanship. Nobody even put their name on the schematic. Perhaps they don't even speak English.
#237
That's a really poorly drawn schematic. I wouldn't be surprised to see "blue wires" that connect the DG212 supply pins to +/-15V.
#238
Did you check those two emitter resistors on the output, R66 and R51? They should be 0.33 Ohm. What is the bias current with no signal?
#239
Schematics and Layouts / Re: hybrid preamp idea
October 11, 2014, 10:44:59 PM
The diodes in the feedback loop of a non-inverting opamp has an interesting characteristic. The output contains the clipped signal that is formed across the diodes with the original signal added in without distortion. Is this why the Tube Screamer is so popular?
#240
Amplifier Discussion / Re: TDA2030 amp
July 28, 2014, 12:41:38 PM
Quote from: Bajaguy on July 28, 2014, 11:40:20 AM
One thing to keep in mind when you are matching a preamp to a chip amp is matching the input impedance of the chip and the output impedance of the preamp. Not paying attention to this will cause some nasty oscillations...... ask me how I know. :grr

Ok, how do you know this? Were you exceeding the input common mode range of the chip? Looking at the ST data sheet for the TDA2030A, the input common mode range is not specified. Ill bet strange things happen when you slam the input rail to rail or try to operate with a non-inverting gain of one.