Designing/Building Pedal with ~2.8kOhms output impedance

[bryant]

Hey all, new to building effects (typically stick with tube amps) but I have a project where I need a pedal to have an output impedance of 2.8kOhms to match the power amp.  The voltage gain needs to be in the ~30-50 range max (I will probably need a volume knob to reduce)...if anyone could point me in right direction with either an available schematic or chip to focus on that would be great.

Thanks,
Bryant

[tonyharker]

You don't have to match the input impedance of your amplifier.  So long that the output impedance of the pedal is 1/10 lower you will be ok.  What amplifier has an input impedance of 2.8k anyway? Most amplifiers have inputs much greater than that. What are you using?

[antonis]

Hi & Welcome..

A CE amp with Collector resistor of 2k8, bypassed Emitter resistor and working current of about 500μA will give a gain of about 50.. - of course, this will result into distorted output for signals of higher than 50mV_p-p amplitude..)
(circuit configuration & bias details may follow under youw wish..)

[bryant]

It's an ICE Power Amp Module.  The low input impedance is what's been tripping me up, so basically thinking I need a buffer or something.  An NE5532 based hi-fi preamp has worked well with it as far as driving the amp, just not thrilled with sound quality (thinking mainly due to tone controls).  Maybe I just need to design around that op-amp. 

[bryant]

Thanks Antonis, that sounds great!

You don't own Dreadbox by chance, do you??  Based out of Athens, think the owner is named Antonis as well.  Longshot I'm sure, lol. 

[R.G.]

As already mentioned, you don't need to match the input impedance of the ICE module. Using a low output impedance buffer to drive it would be a more practical solution.

Impedance matching is used for maximum power transfer. For maximum voltage transfer, as required for signal voltages, you want the impedances MIS matched. You want a much lower output impedance than the input impedance it drives. As a rule of thumb, you want at least a 10 to 1 mismatch. So if you're driving a 2.8K input impedance, you want an output impedance of no more than 280 ohms, preferably lower.

Opamp "output impedance" is funny. The feedback in a typical opamp circuit forces the output to act like a very low impedance  (as in sub-one-ohm) right up until the current limits of the opamp's output stage are reached, when it current limits and starts clipping abruptly.

Most jellybean opamps these days are specified for driving impedances down to 2K at voltages levels as much as their power rails minus a couple of volts on each side. The NE5532 and the LM833 are specified for driving 600 ohms for nearly their entire power supply.

I've considered hooking up an ICE module to an existing preamp, so this is a familiar problem. In my opinion, I would put an opamp buffer in front of the ICE module and consider it part of the power amp. This converts the 2.8K input impedance of the ICE module to whatever the input impedance of the opamp circuit might be. This could be upwards of 100K to a megohm, depending on how the opamp buffer is designed. The opamp gain would be one, perhaps two.

I would then use whatever guitar-specific preamp I liked in front of the opamp+ICE module to get the right gain and EQ. This approach separates the issues of EQ, gain, and impedance matching so you don't have to design them all into one circuit at the same time.

[bryant]

R.G. - thank you for detailed response!  That has actually been my experience and thought process for solving (putting a chip or simple board in front of the module)...I actually already have a tube preamp ready to go but when I connect the 2 there's a severe loss of power...so basically I just need to design a buffer between the tube preamp and ICE Power Amp...but I've never designed with op-amps before, so that's why I posted. 

[antonis]

Designing op-amp buffer isn't (or shouldn't be) rocket science..
But as R.G. already mentioned, are you sure about signal source high output impedance..??

P.S.
No relation with Dreadbox..

[Steben]

RG's suggestion of Opamp input buffer is as simple as it is 100% on topic.

[bryant]

The 2.8kOhms input impedance comes straight from ICE Power's datasheet.  Glad to hear this is simple - does anyone have a recommended op-amp / operating setup to accomplish this? 

[antonis]

A TL072 in non-iverting configuration with feedback resistor of 100k (or better pot) and a shunt cap of 47pF, gain resistor of 2k2 in series with 4.7μF using second op-amp (TL072 is a dual op-amp) for setting bias voltage (as a buffer of 10k/10k divider input) should be fine..
Can't see any need for NE5532 or other bipolar input op-amp other than noise issues..

P.S.
Forgot to mention input cap & bias resistor, of 100nF & 1M respectively..

[FiveseveN]

A picture is worth... 78 words:



More info at http://www.muzique.com/lab/buffers.htm
PS: forgot you need voltage gain.


See the MXR MicroAmp for a full implementation.

[bryant]

THANK YOU!  This should be a very good starting point. 

[PRR]

.......................

[bryant]

OK

[antonis]

Don't know what is happening recently but Paul is getting more and more easily misunderstood..

@bryant: PRR tried to save you a lot of headache by "teasing" you with his own manner, which IMHO had nothing to do with insult/derision/irony..

P.S.
You know, when a new member comes asking for "odd" queries, it's better to firstly verify the true and correct status of his desideratum and them proceed to appropriate suggestions..

[bryant]

 

[Digital Larry]

It's a 100AS1, and there's no suggested interfacing in the datasheet.  Thanks. 

See figure 24.

[bryant]

My mistake - figure 24 has an unspecified op-amp under unspecified operating parameters.  Thanks for your help.