JCM 800 Switch 2 Volumes via MOSFET

[deke99]

Hi all,

I am very new here and have been searching for my answer and have yet to find it, so here is my first post:

I have a JCM 800 2204 clone amp that I would like to add a second Pre-Amp volume control to. So via a footswitch I could select either Preamp Vol 1 or Preamp Vol 2. I have purchased a mojomusicalsupply.com mosfet switching board that controls switching via a ground shunt. I am wondering if this can work or if there is a better option. To use the mosfet ground shunt, I would need to isolate the 2 signals. This is where I am admittedly weak in knowledge. I would think that I could split the signal into the preamp volume by just using a second parallel set of 470K/500pf inputs to the second volume pot and then use the mojo 2n7000 mosfet to short either wiper to ground. I have 2 questions about this setup:

1. In splitting the signal into the 2 volume pots, do I need to adjust the RC preamp input values (470K/500pf) to achieve a better balance of tone? In other words, will the split adversely affect the original tone?

2. How can I isolate and mix the 2 outputs such that the 1 grounded wiper doesn't ground the input to the next gain stage grid?

I've read a fair amount about relays on this board, and perhaps that is the better option here. If so, please feel free to confirm that. I'm looking for the best possible stage tone that I can get, not necessarily to just use what I already bought. I'd much rather throw the $40 mojo board away if there is a more 'toneful' way to achieve the 2 volume switch.

Thank you all in advance for your help!!

Derek

[deke99]

So here is what I'm thinking of trying using the mosfet grounding kit that I already have:

Remove the 470K/500pf RC pair that leads to the pre-amp volume knob. Replace with parallel 100K resistors to split the signal into each of the 2 pre-amp volume knobs.

The wiper will have a mosfet attached that when switched on will short it's wiper to ground. From each wiper I will add a 470K resistor that will lead to the grid of the next gain stage for isolation purposes.

With this arrangement I'd be losing the bright cap for a little darker tone and only adding 100K in series resistance to the signal into the next gain stage. Will this provide enough isolation between the 2 controls?

Thanks!
Derek

[Trace]

With this arrangement I'd be losing the bright cap for a little darker tone and only adding 100K in series resistance to the signal into the next gain stage. Will this provide enough isolation between the 2 controls?

Based on what you are describing you may want to address the input lugs along with the wipers, otherwise I'm not certain it will work in the way you are hoping.


Have fun tweaking
Trace

[deke99]

Hey Trace, thanks for the reply. I'll probably be firing up the soldering iron tomorrow for the mods. Could you possible expand on what you mean exactly by 'addressing the input and wiper lugs'? What am I missing? Thanks!!

[Trace]

Hey Trace, thanks for the reply. I'll probably be firing up the soldering iron tomorrow for the mods. Could you possible expand on what you mean exactly by 'addressing the input and wiper lugs'? What am I missing? Thanks!!

I'm sure someone can explain it better than I (IE: it's 7:55Am and I'm waiting for the coffee to finish brewing--ha, ha) but generally speaking; There is no industry standard for foot switching circuitry and as such there are numerous ways it can be addressed/approached, all with varying degrees of success. There are several ways to go about it and without knowing how you plan to go about it's difficult to predict what the end results will be.

When most try to run two preamp/gain controls in parallel and then ground one wiper while releasing the other, it usually produces less than desired results. There can be no right or wrong when it comes to tone but to minimize negative results, changes in tone, etc perhaps another option would be to also address/look at switching the in lugs as well as the wiper. Again, I'm sure someone here can explain it much better than I. If you would like to post or email a schematic I would be happy to help where ever I can.


Have a great week;
Trace

[deke99]

I understand what you're saying. I have attached a schematic of the relevant portion of the circuit in it's original and split format. The limiting factor in my switch board is that it is a shunt type switch, no series switching. Perhaps I should get a relay board. My main question is really, what will be the affect to the tone by having the signal split by the 2 100K resistors where 1 resistor is always leading to ground and the other is leading to the the gain pot then onto the grid of the second gain stage?

Thanks again for your help, it is very much appreciated!!

[Trace]

I understand what you're saying. I have attached a schematic of the relevant portion of the circuit in it's original and split format. The limiting factor in my switch board is that it is a shunt type switch, no series switching. Perhaps I should get a relay board.

I would recommend using dpdt relays as I think you will be happier with the over all results. Keep in mind that if you rely on a rely to carry/redirect the audio signal (verses using it to shunt a signal to ground) that the tone will only be as good as the quality of the relay's contacts. IE: If you are going to use a relay to carry/redirect audio signal I would recommend sourcing a good quality relay.

My main question is really, what will be the affect to the tone by having the signal split by the 2 100K resistors where 1 resistor is always leading to ground and the other is leading to the the gain pot then onto the grid of the second gain stage?

I can't verbally tell you what you can expect to hear (it's difficult at best to put that into words). If you have the free time I would recommend building both (use the Mosfets first). Once it's up and running you can then disconnect one side and see how it effects/impacts the tone. This will give you a gauge by which to go by in the future.

Having said that I do think relays may be more appropriate for what you are looking to do but that's just one man's opinion. I'm sure others will chime with their opinions shortly.


Have a great Tuesday morning
Trace

[Robert M. Martinelli]

try MAX4601-02-03 series

Hi,
when it comes to audio signal switching without altering the sound in any way I strongly recommend you to use MAX4601-4602-4603 Ics. These have been especially designed for this purpose, are available in NO, NC and NO/NC configurations and ( IMHO ) are currently the state-of-the-art in audio signal switching.

Their "on" resistance is extremely low and remains constant over a wide spectrum of frequencies and they' re voltage operated, making applications very easy to design.

You can find the datasheet and application hints at the Maxim website.
Hope this helps
Best regards
Bob

[Trace]

Hi,
when it comes to audio signal switching without altering the sound in any way I strongly recommend you to use MAX4601-4602-4603 Ics. These have been especially designed for this purpose, are available in NO, NC and NO/NC configurations and ( IMHO ) are currently the state-of-the-art in audio signal switching.

Good point Bob and they do work well with PCB's. The only down side I see in this particular application (correct me if I am wrong) is that IC's can be difficult to work with when hand wiring or mounting onto material (IE: garolite, etc) but they do work well overall. Perhaps you have some tips for hand wiring that would be helpful for the original poster? I wish I did however I only use IC's when mounting them on PCB's.


Have a great day Bob!
Trace

[deke99]

I wired up the mosfet shunt last night and do now have a channel switching amp.

The problem, as many suspected, is that the amp seems to have lost it's balls. I don't notice a ton of interaction between the 2 volume knobs in terms of where each is set, but there is clearly an 'overall interaction' of the circuit that is robbing the tone out of the amp.

Since I changed the RC structure that leads into the second gain stage, I think I will try removing the switching elements and just wire up 1 pre-amp knob with the new configuration to see what difference in tone there is. Perhaps this is a part of the issue. I'll report back and in the mean time I'll take a look at Maxim stuff, thanks for the tip Bob.

[Robert M. Martinelli]

STD mosfets drawbacks

Hi folks,
the problem with the amp "having lost its balls" is most likely related to the way standard mosfets behave when used as audio switches. The main issue with them is the non-linearity of their RDS(on) vs. frequency, which causes alterations in the amp' s bandwidth. The second issue is their RDS(on) is not so low and they can also cause some loss in the signal path. The third issue is noise; their noise figure is usually good but not outstanding, and in this kind of application I would search for the cleanest possible solution. I know using the MAX 4601-02-03 IC is not so straightforward, as they' re pretty tiny devices, but building a small PCB and using a 15W needle-tip soldering iron is probably worth the effort, because they don 't have any of the above mentioned drawbacks. If you still find some signal loss ( I don' think this will be the case, but you never know ) you can throw in a little opamp ( I use NE5534s, OP27s, AD745 and the like ) to regain what loss you have.
Glad to be helpful, only hope my English is still good enough.....
Best regards
Bob

[deke99]

Hey Bob,

Do you have a sample schematic or layout for powering and switching a 4603 chip by chance? I'd be powering the chip using the 6.3V heater supply and I'm not that great at reading data sheets and designing the power supply. And your english is just fine!

Derek

[Robert M. Martinelli]

MAX4603 application hints

Hey Bob,

Do you have a sample schematic or layout for powering and switching a 4603 chip by chance? I'd be powering the chip using the 6.3V heater supply and I'm not that great at reading data sheets and designing the power supply. And your english is just fine!

Derek

Hi Derek,
I don' t have an application note for the 4603, but I can give you some advices on how I would do it using the 6,3 VAC heaters' supply on a tube amp.....

Use a 1/2 or 1 amp bridge ( or four 1N4001....1N4007 ) and a big cap ( say 2200 to 4700 microFarad ) to get DC voltage, this way you' ll get about 8,9 VDC at the cap' s lugs - Use a 78L05 or a 78L06 to get a stabilized voltage of 5 or 6 VDC, ( the L in between means 100 mAmps max current, don' t use standard 7805/06s as they need a minimum load current of about 30 mAmps to operate properly ) which should be enough for our purposes, remember that you can' t exceed this voltage with the input signal ( e.g. if the supply voltage is 6 VDC you can' t switch signals with an amplitude greater than 6 Vpp ). Add two 100 nF caps, one between the 78L05 / 78L06 input and the IC's 0V, and the other between the 78L05 / 78L06 output and the IC's 0V, these caps help damping fast transients the big cap would be unable to shunt to 0 due to its bigger time constant. Connect a 560 Ohm / 0,25 W resistor between the 78L05 / L06 output and 0V to load the IC some ( the resistor will draw about 9-10 mAmps, which will be enough to make the IC operate properly, if the current' s too low the IC won' t be able to stabilize the voltage, and since the 4603 is a mosfet device its current draw is ridiculously low ).

Now let' s have a look to the 4603 - Pin 4 ( V- ) and pin 5 ( GND ) go together to 0V for single supply operation, pins 12 ( Vlogic ) and 13 ( V+ ) go together at +5 ( or 6 ) VDC. The control inputs need to go either to 0 or to V+ to operate their relevant switch, you might want to add a current limiting resistor in series with control inputs to protect them, referring to the 4603 datasheet the typical logic input current is 0,001 microAmps so you can reasonably use high resistor values without spoiling circuit operation, try with 100 K or more, 220K should still be OK.

Hope you won' t have any grounding issue, remember your new 0V from your little PS is not the amp' s GND, if you need to ground it we will have to look at the 6,3 VAC circuit first to avoid problems....

I look forward to hearing from you soon
best of luck
regards
Bob

[deke99]

Thanks for the input Bob, I really appreciate it. I'll work towards getting this circuit together. In the meantime I've been playing around with the split/mix circuit and have found that it is basically my circuit that is bad, not the mosfets that are controlling the switching. With no mosfets installed, the tone still just goes to pot with both volumes wired up and 1 set either very low or off. So I'm off to order parts! Thanks again for all of your help, I really appreciate your in depth response!!

Derek

[Rick Erickson]

[( e.g. if the supply voltage is 6 VDC you can' t switch signals with an amplitude greater than 6 Vpp ). ]

Last time I took signal measurements in a Marshall preamp, and mind you this was back in 1978, I found signal swings in excess of 50V RMS, or ~ 150V P-P. I don't know how you plan to bring the signal level voltage down enough to make it work with a 6V P-P switch, and then bump it back up enough to drive the amp properly. This is and always has been a major stumbling block in tube amps attempting to use solid state switching devices. A relay does not have a "clip threshold", I would go that route.

RE