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Messages - Loudthud

#301
You could use another TDA2030A to split the supply, but why? Most chips you find can't handle the current needed to drive a speaker. Unless your power supply has a center tap, a split supply is more trouble than it is worth. Just use the single supply circuit. The bonus is that you don't have to insulate the TDA2030 from the heatsink. The tab is grounded.
#302
The Newcomer's Forum / Re: jfets
March 05, 2012, 12:16:23 AM
The gate of a JFET is basically a diode to the source and drain. On most data sheets for JFETs, there will be a current rating for the gate, usually between 10mA and 50mA. For an N channel JFET, the gate diode is turned on by positive voltage with respect to the source/drain voltage. So in a normal stomp box with a +9V supply, any input to the gate higher than +9V will turn on the gate diode, probably less in most circuits with gain. All that is necessary to protect the gate for positive inputs is to limit the current to a safe value, usually with a series resistor on the gate. The attached schematics use 68K. 100K or more could be used.

On negative input voltages, the story is different. The gate diode will breakdown usually between 25V and 40V. The diode is not very rugged, it doesn't take very much current at all to destroy the junction. A diode from the gate to a negative voltage (-15V) would work, but in a stompbox, that option is not available. So a zener diode with a signal diode in series or just several signal diodes in series is required. In some circuits where the gate is not at ground potential, but is biased above ground, a diode to ground could be used as long as it doesn't interfere with normal input voltages.
#303
The LM317 is connected wrong. You have Vin and Vout terminals swapped.

Also, the voltage adjust switch should be connected differently. The way you have it connected inbetween selections when you are moving the switch, the output will go to max voltage. Leave the resistors connected and simply short them out with the switch.
#304
One challenge presented by old SS amps is finding the obsolete parts. You can usually find them on ebay, but the price can be outasight. The circuit may accept contemporary parts with a few circuit changes. There was recently a thread on Music-Electronics-Forum where a guy was repairing an old Rhodes piano and had to have the obsolete parts to get the old sound. DTG110B, same part used in some Standell amps. Link: http://music-electronics-forum.com/t27388/ You might have to join up to see attachments.

Pleased to meet you, hope you guess my name....

FYI: I think Keith Richards played bass on that recording.
#305
Those components sense the current flowing in the load and feed it back to the power opamp. This raises the apparent output impedance to the speaker and makes the amp sound more tube like. The capacitor rolls off high frequencies in the current feedback which raises the output impedance higher still.
#306
The Newcomer's Forum / Re: jfets
February 19, 2012, 09:40:57 PM
The J201 is a favorite for stompboxes because it can be used in circuits that require very little in the way of current at low voltages. Typically 0.1mA to 0.5mA at 9V. This translates to battery life of more than a month if you left it operating all the time.

JFETs can vary widely on the amount of current they draw. Usually a 5 to 1 range is all the manufacturer will specify unless you get specially selected parts. The J201 Idss is spec'd at 0.2mA to 1mA. The generic J300 specifies the Idss current at 4mA to 45mA. That's over an 11 to 1 range. There are tighter spec'd units, suffix A, B, C and D. J300A: 4mA to 9mA, J300B: 7mA to 15mA, J300C: 12mA to 25mA, and J300D: 21mA to 45mA. Do your parts have one of these suffixes?

To operate at 9V, the J300 will have to opreate at significantly higher current. At that current, battery life could be only several hours. Another option for the J300 would be to operate it with two 9V batteries connected in series. That will help reduce the current. If you will be using an external power supply, that won't be a problem. How do you intend to power this device?

To make the J300 work in that circuit, two resistors will have to be changed to customize the circuit for your J300s. The J300s will need to need to be tested to determine those resistor values. These tests will require you to connect the FET in a simple circuit with one or two resistors and measure a voltage or two. If you want, I will walk you through this process.

Note: The circuit you posted has no protection on the input. A simple static discharge could kill the FET. Also, if a J300 is operated at a high current, it can pickup any nearby radio station, even in the FM band. Some simple changes to the input circuit of the FET would be wise.
#307
I don't see anywhere that the +9V is connected to the board. Should be a connector of some sort or a place to solder a couple of wires. There does not look like there is enough clearance around the mounting holes in each corner. There should probably be 4 or 5 mm from the center of each hole. Are you planning on mounting the pots to the PCB? You will need bigger pads, probably twice as big as the ones you have now.
#308
The Noisy Cricket is tweeked to be a guitar amp. It has pretty limited low frequency response and lots of gain. A few changes would make it more bass friendly. Replacing the LM386 with an opamp would give lower noise and a little cleaner sound (as a preamp) but not a night and day difference.
#309
Amplifier Discussion / Re: marshall 5210 problem
January 13, 2012, 02:15:09 PM
Seems like there is a power amp problem. Do you get any sound with the reverb turned up when you give the pan a thump? Have you tried running the Effects loop out to another amp? That will tell you if the preamp is working.
#310
Amplifier Discussion / Re: Noisy Sunn Coliseum 300
January 12, 2012, 02:46:55 PM
The CMOS clipper stage just softly pre-clips the waveform before it's sent to the power amp. The power amp does not have enough gain to clip the signal any harder. If the power supplys dip because of currrent draw or low line voltage, the supply to the CMOS also dips so the soft clipping is maintained. The trimpot R101 allows a slight adjustment to the CMOS supplys.
#311
Amplifier Discussion / Re: Noisy Sunn Coliseum 300
January 10, 2012, 09:51:14 PM
The CMOS stage is what gives the amp the soft clipping. Note how the -48V is used to derive the supply for the CMOS so the CMOS clipping level tracks the power amp sag.

What happends to the noise level when a cable is inserted into the Power Amp Input jack? That disconnects the preamp but it is before the CMOS stage so if the CMOS stage is the source of noise, the noise will still be high.
#312
Amplifier Discussion / Re: Fahey Amplifiers
December 31, 2011, 05:54:43 PM
Juan, I'm very impressed by your product and your company. Can you post a pic of the inside of the amp?

I'm curious about some of the changes that have occured over the years. Did any versions sound better or worse than others? I'm asking in reference to power amp changes, but preamp or speaker changes are also of interest. Any reliability problems that caused circuit or component changes?
#313
Schematics and Layouts / Re: SUNN Stage Lead power issue
December 30, 2011, 05:49:31 PM
Quote from: markorock37 on December 30, 2011, 01:56:41 PM
For whatever reason, pins 11 and 14 are at 24V now with the transistors removed.
That's what you should expect.
Quote from: markorock37 on December 30, 2011, 01:56:41 PMOne of the new transistors is bad, and one of the originals was bad too.

There is the possibility that something in the circuit killed one of the new ones when you installed it. Did you find a short when you checked the transistors with an ohm meter? Check for shorts from the case of those output transistors to the chassis before you power up the amp. This is a good time to have one of those light bulb limiters, it will usually save the transistors if there is still a problem.

Nobody gave you an answer before, but the MJ15003 is a good substitute for 2N3055.
#314
Schematics and Layouts / Re: SUNN Stage Lead power issue
December 27, 2011, 08:55:40 PM
Quote from: markorock37 on December 27, 2011, 06:45:21 PM
A bit more testing I found about 10V at pins 11 and 14 from driver transformer to Q2. Pins 12 and 13 from driver transformer to Q1 is less than a volt. Sounds like the driver transformer is bad to me. Its a model 28-4300

10V is a little low. Pins 11 and 14 should be at half the supply voltage (+47). They should go to that voltage when Q1 and Q2 are removed and should be about the same when a good pair of transistors are installed. You can see that R128, R137, R136 and R139 form a voltage divider. Some current goes through R132 but not much. C1 could be bad, but it wouldn't affect the voltage when no speaker is connected.

These transformers develop shorts between windings. Disconnect 11, 12, 13 and 14 and then check that those wires measure infinite ohms to 15 and 16 and to ground. Mercury Magnetic$ makes the only replacement transformer available. If you find a short, there are two more windings on the transformer that were not used in this amp. They probably just cut the wires off. Any chance you can see them and maybe solder a wire to them? One pair is Red/Orange, the other is Yellow/Green.

Do you know how to check transistors out of circuit with an ohm meter? Basically you should see a diode between base and collector and another diode between base and emitter. Collector to emitter should show a high resistance or open.

#315
Schematics and Layouts / Re: SUNN Stage Lead power issue
December 25, 2011, 10:25:52 PM
This should help. The schematic and layout from the Studio Lead which should be the same as what you have. What are the numbers on the power transformer? The Studio Lead used 28-1650.