Prototypers wanted

[n_maher]

Home Depot / Lowe's carries rubber bumper feet that are pretty indistinguishable from the stock Hammond feet.

[Pars]

Doug (or anyone):

Will the bias imbalance between channels that I am noticing at PL/PR have any negative effects on performance? I am guessing that a better matched tube pair is the only way to fix this? Or is it a non-issue?

[n_maher]

I'd do a quick check of output amplitude to see if it's affecting performance at that level. Otherwise you're just chasing rainbows hoping for tubes to be better matched than 10%, at least IMO.

[Pars]

Thanks Nate. I'll run RMAA on it, and also some test tones and look with joe scope to see how things are doing. I would certainly expect sand state to be better than this j/k

[dsavitsk]

Hard to know whether the imbalance will matter. Yours do seem a little more off than most I have used. Ideally, you can find a pair of tubes that are matched for both mu and gm. But, the way to do this is to put them in the circuit, run a test tone through it, and measure the output. A 60Hz tone and a basic multimeter measuring AC is the easiest way to do this.

[dsavitsk]

A couple of the questions here suggested that a more thorough explanation of this amp might be in order. So, to that end I wrote up a little primer on parafeed amps for headphones. This is still a bit of a rough draft, but I posted it in case it is helpful to anyone. Comments on where I might clarify things are appreciated.

http://www.ecpaudio.com/pdf/parafeed_basics.pdf

[Pars]

Thanks, that is helpful.

It has probably been stated in the thread already, but which switch position is which? If from the schematic, pin 1/4 of the switch should be the higher impedance and pin 3/6 the lower? For this amp, is it 300 ohm and 32 ohm taps?

[mugdecoffee]

A couple of the questions here suggested that a more thorough explanation of this amp might be in order. So, to that end I wrote up a little primer on parafeed amps for headphones. This is still a bit of a rough draft, but I posted it in case it is helpful to anyone. Comments on where I might clarify things are appreciated.

http://www.ecpaudio.com/pdf/parafeed_basics.pdf

That article was immensely helpful for me. I fully understand resistance/current/power/etc but tubes still give me a hard time. Your explanations definitely filled in some holes.

Few things I still find confusing/have questions about:

-For grid voltage vs. plate current, the way I thought tubes worked is that the electrons glow off of the cathode, go past the grid, and hit the plate. This would make current flowing from plate to grid. Also, I thought that a more negative grid would repel the electrons flying off the cathode thus lessening current. In your explanation, both these directions are backwards. Am I missing something, maybe something to do with electron flow vs current flow?

-What determines the rp? Can it be modeled as a resistor somewhere in the way that output resistance can? I can't quite picture where the resistance is in the circuit.

-How does a choke swing voltages over the PSU voltage? I guess this question is more in a physical sense.

[dsavitsk]

-For grid voltage vs. plate current, the way I thought tubes worked is that the electrons glow off of the cathode, go past the grid, and hit the plate. This would make current flowing from plate to grid.

This is just symantecs -- I consider the flow of electrons to be -= to the flow of current. In solid state world, it is, as you say, the opposite.

Also, I thought that a more negative grid would repel the electrons flying off the cathode thus lessening current. In your explanation, both these directions are backwards. Am I missing something, maybe something to do with electron flow vs current flow?

Bah -- this is what proofreaders are for -- I said it exactly backwards

-What determines the rp? Can it be modeled as a resistor somewhere in the way that output resistance can? I can't quite picture where the resistance is in the circuit.

You can indeed think of it as an inherent resistance in the tube. In phono stages where you need a certain rp for filter purposes, it is common to augment the rp with an additional resistor.

-How does a choke swing voltages over the PSU voltage? I guess this question is more in a physical sense.

That, I'm afraid, it way too big of a topic for here, and something I can't help much with -- you''ll need to read up on transformer theory.

[dsavitsk]

up is 300, down is 32.

Article updated

[Beefy]

A couple of the questions here suggested that a more thorough explanation of this amp might be in order. So, to that end I wrote up a little primer on parafeed amps for headphones. This is still a bit of a rough draft, but I posted it in case it is helpful to anyone. Comments on where I might clarify things are appreciated.

http://www.ecpaudio.com/pdf/parafeed_basics.pdf

Very handy

Just one suggestion for the diagrams..... it may be helpful to show the parallel loads on the tubes a bit earlier in the document. It didn't really 'click' for me until I saw both the CCS and the parafeed cap/transformer connected to the plate.

[Pars]

Is there any particular heatsink grease anyone would recommend? I used a silpad on the regulator, but only had some really old ratshack thermal grease for the BJTs. Arctic Silver? Other?

[Pars]

Also, I got some extra end panels, so if anyone wants one for redos, let me know.

[luvdunhill]

though there is no reason to not turn the dial up to 70 or 100%

I'm finding this to be the case more and more. I've even been listing to this with the K1000.

[luvdunhill]

A couple of the questions here suggested that a more thorough explanation of this amp might be in order. So, to that end I wrote up a little primer on parafeed amps for headphones. This is still a bit of a rough draft, but I posted it in case it is helpful to anyone. Comments on where I might clarify things are appreciated.

http://www.ecpaudio.com/pdf/parafeed_basics.pdf

perhaps add a small section about cathode connection options, given that this might be linked to in conjunction with this amp that can select between WE and LED options?

[dsavitsk]

perhaps add a small section about cathode connection options, given that this might be linked to in conjunction with this amp that can select between WE and LED options?

For this amp, unless someone has a wildly different take on it than me, I think we are going to do away with the WE option. I think it sounds (and measures) worse. But, as for the larger comment, I was leaving biasing out of the article as it was only meant to deal with parafeed specific issues. Maybe I'll do a separate short article on biasing issues, though there seems to be plenty out there on the topic.

[cetoole]

F I think it sounds (and measures) worse.

Measures worse in what way?

[Pars]

I started running RMAA on the amp tonight. There does seem to be a channel imbalance with the higher bias tube having a slightly higher output. The delta appears to be greater at low impedance. I have a load box (Amb's design) that I was using, but also ran unloaded measurements. I am using an M-Audio Firewire Audiophile box for the test. I was adjusting the input levels (I assume coming out of the amp) with the M-Audio control panel and had to compensate for the level imbalance. Not sure whether I should do that or just lock them at an equal position.

The CCS 75 ohm resistors are Draloric 0.5%, but I did not measure or match these. I'll take some voltage measurements of the drops across the L and R, as well as the LED voltages tomorrow. I'll try to pdf the RMAA test results and post tomorrow also.

[dsavitsk]

When you are dealing with tubes that might be +/- 20% and transistors that could be worse, matching resistors hardly matters. If you do want to match anything, tubes would be first followed by Q1L and Q1R, and really matching Q's is not important at all.

However, we are recommending that R4L and R4R be closer to 56 ohms instead of 75 which will boost the current through the tubes quite a bit -- up to around 18mA. It measures better, and Tom says it sounds a lot better, particularly on high Z phones for some reason. If you don't want to pry the 75R resistors out, just bypass (parallel) them with something around 200R.

To determine more specifically what resistor to use, just divide 1 by the current you want. 1 is approximate here, and depends on how close Vbe is to 0.7, and how close the drop across the CCS's biasing LED is to 1.7 (notice that 1.7-0.7 = 1).

As for measurements, these transformers are not likely to be inductive enough that they will give very much bass when run into a high Z load like a sound card. Though, I don't have any idea with the primary inductance is. But, assuming it is not sure high, the high load will likely give you better than expected results at mid to high frequencies, and worse than expected results at low ones.

@Colin - THD was definitely better with the LED. I didn't save any of the measurements I did, and it was a while ago, so I'll have to redo them at some point. Overall sound was flat and bass was really flabby and weak. Ryan listened to it for about 8 seconds before declaring the WE option inferior -- it was not a subtle difference. I can't say, though, if this is just this amp, or if this is a generalizable result. I have a DAC that runs an AD1865 into the grid of WE connected, trafo coupled, C3g's and it works fine there. Maybe it would work better with LED bias, though?

[Pars]

I thought these tubes were matched? As for increasing the bias, while I was running RMAA, the temps on the BJTs were ~60 deg C. The voltage regulator was around 55 deg C, heatsink was uncomfortable to touch, but case wasn't hot anywhere I felt (top is not on or drilled currently). It's no problem for me to replace the 75 ohm resistors, though I'll have to see what I have laying around. Off the top of my head, I have some ratshack carbons (I think) from a variety pack I may have some RN60s or something around also, or as you say, I can parallel something. I could put 100 ohm pots in also, but since this isn't intended to be a play amp, probably not.

Again, if anyone needs another end panel or two, you can have them. Out of the US I might have to charge postage, but free otherwise.

[TomB]

I thought these tubes were matched? As for increasing the bias, while I was running RMAA, the temps on the BJTs were ~60 deg C. The voltage regulator was around 55 deg C, heatsink was uncomfortable to touch, but case wasn't hot anywhere I felt (top is not on or drilled currently). It's no problem for me to replace the 75 ohm resistors, though I'll have to see what I have laying around. Off the top of my head, I have some ratshack carbons (I think) from a variety pack I may have some RN60s or something around also, or as you say, I can parallel something. I could put 100 ohm pots in also, but since this isn't intended to be a play amp, probably not.

Again, if anyone needs another end panel or two, you can have them. Out of the US I might have to charge postage, but free otherwise.

My apologies on the tubes. I'll send you a couple more.

What happens is that some tubes may break-in with the few minutes/hours of use in an amp and the output could be quite different than when tested. That doesn't happen very often, but it does happen. Some say it's bits of gas that infiltrate over the 30-40 years of sitting on a shelf. A few hours of running in an amp allows the getter to burn out the gas, changing the characteristic of the tube. Then again, the tester may have just blown it.

I can confirm what Doug says - the amp has always been optimized toward Grados and low impedance cans, but running the CCS at the different current made a huge difference with mid-to-high impedance phones. Mine has 3.8uf coupling caps and I've read where that should produce a bass hump of sorts. To tell the truth, I never noticed until making this change with the CCS (I soldered 221R resistors onto the 75R of R4L and R4R). Now, my little HD25-1's sound like they have a subwoofer. HD600/580's sound remarkably better now and the impedance switch has a noticeable effect - much more "forwardness" with the Senns than at the lower switch setting, if that makes any sense.

[Pars]

Thanks Tom.

Is there an optimal voltage, or does higher = better in this case (measured at PR / PL to ground)?

[dsavitsk]

I thought these tubes were matched? As for increasing the bias, while I was running RMAA, the temps on the BJTs were ~60 deg C.

As the current in the CCSes is increased, the amount of voltage they have to drop decreases both because the plate's voltage increases, and because B+ sags a little. So, the heat will not necessarilly go up too much.

I would not use a trimmer, at least not one that is adjusted by hand. These resistors are at high voltage, and the reason we didn't want a trimmer was to keep fingers out. Indeed, there are better CCS designs, but they rely on mosfet Vgs to set current which is a lot less predictable than BJT Vbe and would thus require a trimmer. We decided that it was better to be safe here. Plus, trimmer's can move when they heat up. In the big version of this amp, I use mosfets that are very closely matched.

The voltage regulator was around 55 deg C, heatsink was uncomfortable to touch, but case wasn't hot anywhere I felt (top is not on or drilled currently).

We are going to increase the HS size in the final (boh for BJTs and for the reg). But, I don't think an IC running at 55 or 60C is a big deal.

[dsavitsk]

Is there an optimal voltage, or does higher = better in this case (measured at PR / PL to ground)?

Hard to know. Higher might mean a better tube?

b/t/w/, looking at the curves again, increasing the cathode bias with a 3 to 3.3V led might be a good thing. It looks like it is a little more linear there.

[Pars]

Agreed on temp... 55-60 deg. C seems fine. When they get up to 70 I start getting concerned. Once I put the top on, I may need to do something.

The power transformer does not feel warm or measure above 35 or so. I may use the heatsink for the top over the BJT area instead.