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Single supply alternative to a CMOY (sugestions please)

post #1 of 4
11/4/11 at 6:31am
Thread Starter 

So all I need is a deck, my battery preamp, my cans and a flat surface; I'd like to build a battery powered Phono-Pre-amp-inc-headphone-amp

Essentially based on the Le Pacific phono pre but I'd like to have in the same housing a decent headphone amplifier that can nicely drive my Alessandro MS1's (Grado SR-80).

 

At first I thought Pacific + CMOY in the same box, nice; but when I thought about it more I realized the Pacific is a single supply 0/24VDC circuit and the CMOY is a +/- Supply. I don't think I can power the two from the same battery pack. So I'm looking for either a solution to the above or a decent alternative to the CMOY which can run on a single supply without killing the battery. the only circuit I've found so far is this JFET-MOSFET arrangment but I reacon just by looking at the heatsinks that will eat batteries!

 

Does anyone have any circuit sugestions I could look at?

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post #2 of 4
11/4/11 at 9:53am

The Cmoy can be configured for "true" +/- voltage supply, or more commonly uses a virtual ground arrangement. 

 

With a virtual ground you *should* be able to get both to run off of a single supply, but take care NEVER to confuse your grounds. This is very important if the Cmoy is going to be used as a preamp as well as a headphone amp (am I correct in assuming this is for a DJ application?) If its JUST a headphone amp just make sure to isolate everything. 

 

As another idea, why not build an op-amp based phono stage using a "real" +/- supply for both the phono stage and the Cmoy? Im not totally thrilled with feedback riaa, and passive RIAA with opamps does not cost much more (cost in dollars, battery consumption, or physical size). Compared to the idle current of those fet's its all about the same really. Just a thought. 

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post #3 of 4
11/4/11 at 2:43pm

Could try something like this...

 

invert_invert.jpg

 

...un-simulated, untested.

 

An inverting opamp with it's bias set to 1/2 Vcc by R5, R6 has a gain of ~ 5 and its B/W limited to ~34kHz. The output is AC coupled to a log pot volume control. The wiper is itself AC coupled to an inverting buffer current drive opamp with its bias set by the same network which is shared by all the opamps. Volume controls usually operate round about 30%, so the impedance out is <=1/10 of the input at the driver stage. Both RC filters (C1, RV1: C11,R3) viewed in isolation roll of at ~1.6Hz. The two caps ensure no DC flows in RV1. The output electrolytic is chosen overlarge to keep the distortion it causes to a minimum. The dual inversion conserves polarity. Resistor values are kept as low as practicable to minimise noise. The inverting configuration is actually better in terms of distortion of the individual opamps but doesn't lend itself as readily to global feedback, the output cap is the fly in the ointment.

 

Don't forget those caps to be distributed to the chips at the power terminals.

 

w

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post #4 of 4
11/5/11 at 3:31am
Thread Starter 

Cheers for the Feedback guys
 

Quote:
Originally Posted by nikongod View PostAs another idea, why not build an op-amp based phono stage using a "real" +/- supply for both the phono stage and the Cmoy? Im not totally thrilled with feedback riaa, and passive RIAA with opamps does not cost much more (cost in dollars, battery consumption, or physical size). Compared to the idle current of those fet's its all about the same really. Just a thought. 

 

I had this thought also, but I really like the JFET Phono pre and I'm keen to build another just for a different application.



Quote:
Originally Posted by wakibaki View Post

Could try something like this...

 

invert_invert.jpg

 

...un-simulated, untested.

 

An inverting opamp with it's bias set to 1/2 Vcc by R5, R6 has a gain of ~ 5 and its B/W limited to ~34kHz. The output is AC coupled to a log pot volume control. The wiper is itself AC coupled to an inverting buffer current drive opamp with its bias set by the same network which is shared by all the opamps. Volume controls usually operate round about 30%, so the impedance out is <=1/10 of the input at the driver stage. Both RC filters (C1, RV1: C11,R3) viewed in isolation roll of at ~1.6Hz. The two caps ensure no DC flows in RV1. The output electrolytic is chosen overlarge to keep the distortion it causes to a minimum. The dual inversion conserves polarity. Resistor values are kept as low as practicable to minimise noise. The inverting configuration is actually better in terms of distortion of the individual opamps but doesn't lend itself as readily to global feedback, the output cap is the fly in the ointment.

 

Don't forget those caps to be distributed to the chips at the power terminals.

 

w


Thanks WB; I wish I had time to experiment a bit more with this sort of thing; I'm not much good at design (I've designed a few distortion pedals in my time, not always Deliberate) but I see no reason why that wouldnt work great.

 

I'm quite interested in a lazy chip-amp solution if one exists; I've just noticed on the NS website that they produce a range of headphone amplifier chips; the LM48XX.... for phones and MP3 players etc. I'm sure most of them are the sort of underpowered drivers we build amps to replace in the first place. But are any of them are actually any good for diving big cans well???

 

I might start a new thread to ask the question.

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