Quickie + CMOY for Headphones?

[russtafarian]

I’m thinking of adding a CMOY type op-amp circuit to the output of my Quickie in place of the Speco transformers to better drive my headphones. 

Since the op-amp I’ll use can handle +/-18vdc, can the 36v batteries be used to drive the virtual ground PS of the CMOY? 

Will this result in a huge current drain on the batteries?  I’m willing to give some battery life to power the Quickie and the op-amp from the same supply, but not if battery life drops to just a few hours.

Will I need another power switch for 36v supply to keep the op-amp from drawing current when the Quickie power switch is off.  Would substituting a 4 pole switch for the 3 pole switch work?  Thanks.

Russ

CMOY schematic: http://tangentsoft.net/audio/cmoy-tutorial/misc/cmoy-tangent-sch.pdf

[Grainger49]

You should check the specs on the Op-amp you plan to use.  Unless it is like the GainClone an Op-amp, even in class A biasing, shouldn't draw a huge current.

Can you post a link to the chip you propose to use?

[russtafarian]

The op amp will draw less than 10ma of current.  Since there's lots of room under the hood, it's no big deal to set the op amp up with its own batteries and power switch.

[Caucasian Blackplate]

If one was really ballsy, you could add another pair of 3S4's (with 2 more d cells), then use the 2nd two tubes as cathode followers and add some global feedback to get the output impedance down.

That, however, would necessitate a bigger chassis plate.

[Paul Joppa]

The 3S4 cannot produce much signal current, so lowering the source impedance with a cathode follower still won't drive a headphone well. To get useful headphone output with just the tube, you need an output transformer, matched to the headphone impedance.

The popular Quickie/Speco combination will generate about 1mW into a 300 ohm headphone but only 10%-20% as much into a 30 ohm load - 7 to 10dB less. Using parafeed you can increase that to four times the power (another 6dB), but in addition to the choke, the Speco will now need to see 600 ohms optimally, and the operating point would have to be adjusted a bit.

While I have a transformer design in mind for headphone amps, it's not likely to materialize until some current projects are in production. Meanwhile, if anyone is interested, look around for a transformer with an 8K primary (I think 5K to 10K is a reasonable range) and a secondary matched to your headphones.

[russtafarian]

I finished this up last night, and so far I like the result.  I used two LME49710 metal can single op amps in the circuit, chips that are much loved by the head-fi crowd.  I measured the current draw for both at about 11ma.

I do give up a bit of the spacious, etherial sound of the Speco'd Quickie, but I gain a lot in headphone drive and control.  This thing now has bass!  So the Quickie+CMOY makes a nice "hybrid" headphone amp.  More lush and open than a straight CMOY, and better low impedance current drive than a Speco'd Quickie.

Russ

[JC]

I'm curious, how did you end up powering the op-amp portion of the circuit?

[russtafarian]

I'm powering the op amps with two separate 9v batteries and a separate power switch.  I may try some other configurations.

[russtafarian]

I did some more work on this over the weekend.  I changed the Quickie coupling capacitors to .1uf WIMAs and output resistors to 100k to match the input circuit of the CMOY.  I also doubled the voltage going to the op amps by using four 9v to get +/-18v. 

I can't tell you how thrilled I am with this combination.  The Quickie-CMOY combo has brought a high level of clarity and control to my headphone listening.  Highly recommended if you use a quickie for headphones.

[JC]

I'm curious: Did you happen to measure the current draw of the op-amp section after you increased the supply Voltages?

[russtafarian]

I didn't remeasure the current draw after doubling the voltage to the Opamps.  Since I use it at work, I'll bring my meter in for a quick check. 

[JC]

Any curious co-workers yet, asking about those funny glass things sticking out of the top?

[grufti]

Careful with your supply voltage for the LME49710. National specs the supply voltage as: Power Supply Voltage Range ±2.5V to ±17V. The Absolute Maximum Ratings section has this to say: Power Supply Voltage (VS = V+ - V-) 36V. Your batteries might give you close to +/-19.5V when they are fresh. You could be nearly 10% above the absolute maximum value.


"I also doubled the voltage going to the op amps by using four 9v to get +/-18v."

[russtafarian]

"Careful with your supply voltage for the LME49710."

Yeah, I know I'm pushing the LMEs a bit over the edge.  I figured since I'm using batteries I don't have to worry about significant voltage spikes like I might get from an AC supply.  I'm taking a chance, but so far it's paying off sonically.  I was surprised at how much cleaner my phones sound after doubling the voltage.

[russtafarian]

"Any curious co-workers yet, asking about those funny glass things sticking out of the top?"

A couple people have asked about it.  They think the tubes are a cosmetic gimmick.  A co-worker's husband is an antique radio collector so the Quickie immediately caught her attention.  I told her she could take it home for a weekend so her husband could play with it.

What most folks comment on is the funky mauve & beige color scheme of the ATH -AD700 phones.  I like them for work because they're open air so I can hear what's going on around me, light and comfortable, and resolving enough to satisfy my tune-lust.  But inconspicuous they are not.