Signet EP500 cans

[alanh]

Does anyone have any data on these things? I got them 20 years ago or so and liked them then. Recently I've gotten back into cans (quiet listening after the kids are in bed will do that to you). I have a Quickie wired with Hammond 125ESE at 10K:32 in parafeed with the PJCCS for headphone listening. Now they sound a little anemic--I do understand that the Quickie, output transformer notwithstanding, might not be a great match for them. Until I pull the trigger on a new set I want to know the impedance of the Signet EP500s I have.

Thanks.

Alan

[Paul Joppa]

Google gave me this: http://www.adelcom.net/SignetHP.htm

Three observations:

1) The Hammond 125ESE has a minimum inductance of 9.58 henries, according to the Hammond specs. That has a reactance of 10K at 166 Hz, below which the Quickie will lose the ability to achieve full power. It is 4K at 66Hz, below which even the small-signal level will fall. That may be the reason for your "anemic" sound.

2) The sensitivity is 92dB/mW. Quickie with CS load can make no more than a couple mW into a 10K load impedance, so more sensitive phones will probably be less anemic.

3) The only impedance information given is "...matching impedance: 4 to 16 ohms". You might try a lower impedance tap such as 8 or 16 ohms to see if that makes a positive difference.

Once we identify a suitable output transformer, I'll develop a set of mods to optimize headphone performance. Quickie was after all designed as a preamp, not a headphone amp, and the configurations in use are the product of Forum members experimenting - not a design optimization. The first mod will be to correct the error in the manual, and get the D-cell filament batteries in the right orientation!  :^)

[alanh]

Paul,

I found that site too. The impedance range confused me. I'm not sure why I didn't try a different tap first--probably wishful thinking.

To be sure, the Quickie sound is "a little" anemic--I like it better than the two other (cheap) head amps I have in the house but find myself looking for a little more oomph on some recordings. No complaints, of course. I understand the limitations of the Quickie as a HP amp, and of trying to adequately control a low-impedance load with an output stage that only draws 2 mA.

I understand what you're saying about the reactance of the 125ESE's inductance. If I truly understand, my initial thought of replacing the current source with a 150H choke wouldn't make any difference.

Alan

[Paul Joppa]

If you stay with the 125ESE, just use it as a series feed transformer. Its low inductance is caused by the fact that its core is airgapped (it's rated for up to 80mA!) so why not take advantage of that? Its DC resistance will be much less than 4K, resulting in greater voltage on the 3S5 plate, and higher output power. Between that, and finding the optimum secondary winding impedance, you should get a noticeable increase in available power.

[SteveH]

Once we identify a suitable output transformer, I'll develop a set of mods to optimize headphone performance. Quickie was after all designed as a preamp, not a headphone amp, and the configurations in use are the product of Forum members experimenting - not a design optimization. The first mod will be to correct the error in the manual, and get the D-cell filament batteries in the right orientation!  :^)

I have to laugh, I bought a Quickie two years ago and have finally dragged the kit out so that I can put it together and the first thing I did was go looking for info on the D-cell battery wiring 'issue' so I could update my instruction manual... ;^)

Mods to optimize headphone performance would be great!

SteveH

[alanh]

Series feed...that was my initial thought. In fact, I had it wired that way for a bit and there probably was more power. In the end, however, I wanted to keep your fine current source board in the circuit to compensate for voltage sag as the batteries age.

Alan

[Paul Joppa]

Series feed...that was my initial thought. In fact, I had it wired that way for a bit and there probably was more power. In the end, however, I wanted to keep your fine current source board in the circuit to compensate for voltage sag as the batteries age.

Alan

The cathode resistor provides an equivalent voltage-sag compensation for series feed - my apologies for not making that more clear. Covering ALL the ramifications is pretty hard! The current source eliminates any effects of the plate choke in parafeed, leaving only the output transformer effects - but series feed should be the same as long as the transformer is gapped for plenty of current. And 80mA is a pretty good margin relative to 2mA!   :^)

The virtues of parallel feed are mainly two: power supply isolation, and the possibility of interleaved output transformers. With batteries, there is no power supply noise (*but see below) and as you already have an airgapped transformer you might as well give it a fair shot.

The problem with current source loads is that they consume some of the power supply voltage - around 1/3 of it, which translates to half the output power potential if you are operating under restricted voltage conditions, as with Quickie. Great for a low distortion preamp, but limiting for headphone application unless you have plenty of power available (like the Smack).

==================

* Some observers have claimed that batteries create noise as they discharge, and that they have an AC impedance that is not close enough to zero to be inaudible. You can parallel the battery with a good capacitor to test this - if the capacitor impedance is less than the battery, the signal current will go through the capacitor instead of the battery. I would put the capacitor after the power switch so that leakage current does not slowly drain the battery, but it won't make much difference unless you have long idle periods. If anyone tries this, please report your results! (The capacitor should be larger than the output or parafeed capacitor in order to help make it an unequivocal test, but even a smaller cap should have an effect in the high frequencies. If you have a few snazzy caps in your junk box, it's a free experiment!)

[alanh]

First off, it's been said before, but many thanks to Paul J who patiently reads just about every post here and helps us get the best out of our BH kits (even if we're trying to do something with a BH product that it was never intended to do). Like (I suspect) most here, I've done plenty of reading about tube amp theory but have limited time to devote to my hobby. Consequently I lack experience. This forum has helped me make my experimenting much more efficient.

Now on to my Quickie headphone amp mods. Per PJ's suggestion, I have two Hammond 125ESE wired as traditional SE output transformers, 10K:8. I haven't heard the other BH headphone amps, but this configuration doesn't sound half bad to me. Obviously, the volume control is just about maxed out into 8 ohms, but it's loud enough and has adequate bass for me. (And no, I didn't purchase the transformers for this purpose; I had them for other SE experiments.)

Sorry this thread has been mostly about Quickie instead of headphones. In summary, the Signet EP500 is probably something like 8 ohms.

My next question is about higher-impedance phones. I'll start a new thread for that one.