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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.