A tube headphone amplifier using 6080

Hi,

Here is a headphone amplifier using the 6080's tubes.
Before X1 connector there is a balanced stepped attenuator.
Driver stage mounts the 6CG7, each rail has a voltage gain of 11 times (20.9dB).
With 12dBu at the input (8.73Vpp) we have about 80Vpp at the 6080's grids.
6080 working point is at Eb=105V, Ec=-40V, Ia=60mA, and the voltage swing at the anode of each 6080 is about 97V peak to peak, or 34Vrms.
At the secondary of T1, which has a ratio of 8.8:1, 2K5 prim. and 32 ohms sec., we have 3.89 Vrms developping on the 32-ohm load, thus a power of 472mW, I believe enough to drive low impedance headphones.
Any comment/suggestion highly appreciated.

> 472mW, I believe enough to drive low impedance headphones

Enough to blow your brains out your butt.

As a rough guide, headphones need 1/1000 of the power of speakers.

So 472mW in phones is like a 400+Watt speaker amp (in home use).

People do 400W for speakers, and a 400mW headphone monster is far cheaper.

I didn't go through your design. +12dBu input seems high, but since that gives brain-blowing power and typical levels will be under 0dBu, it's fine. A push-pull 6080 "can" give ~8 Watts, and in any case eats 15 Watts heater power, and your amp pulls 40 Watts DC, so a half-watt output seems inefficient.

Do C1 R7 PR1 R8 C2 actually do anything? 68K/250= max gain of 270. 6CG7 gives gain less than 16, 6080 gives gain less than 2.8, more like 1.2 in this case. 15*1.2= raw gain of 18, NFB won't let the gain be more than 270, the NFB is doing nothing at all. Waste of 3 dollars. Anyway the amplifier will be pretty linear at high levels, and very linear at safe headphone level.

6080 is a really tame lame audio tube. Drive is insanely high, linearity is poor (and varies depending on factory and vintage). It was made as a pass-tube, though I think it isn't as good for that as it could have been. It has also been reported that the matching between sides is not good enough to trust a common cathode resistor (in fact RCA sais this when it was introduced).

2A3 is a much better audio tube. Don't like current prices? Get 6EM8/13EM8: it's darn-near a small 2A3 with indirect heat, and as a bonus you get a 12AT7-like driver.

If you can find a somewhat higher load transformer, the darn 6CG7 will make all the audio power most headphones will ever need.

But if you want a very expensive heater which also drives headphones with authority, do the 6080.

> Any comment/suggestion highly appreciated.

[quote author="PRR"]> 472mW, I believe enough to drive low impedance headphones

Enough to blow your brains out your butt. [/quote]

Ah ah

I didn't go through your design. +12dBu input seems high, but since that gives brain-blowing power and typical levels will be under 0dBu, it's fine.

Click to expand...

I just took 12dBu for comparison since these kind of levels are common from professional sound card outputs, such as 192 cards of P.T.

[quote author="PRR"]Do C1 R7 PR1 R8 C2 actually do anything? 68K/250= max gain of 270. 6CG7 gives gain less than 16, 6080 gives gain less than 2.8, more like 1.2 in this case. 15*1.2= raw gain of 18, NFB won't let the gain be more than 270, the NFB is doing nothing at all. Waste of 3 dollars. Anyway the amplifier will be pretty linear at high levels, and very linear at safe headphone level. [/quote]

Oh, NFB, I added it to linearize the response of the amp. No need for that?

[quote author="PRR"]6080 is a really tame lame audio tube. Drive is insanely high, linearity is poor (and varies depending on factory and vintage). It was made as a pass-tube, though I think it isn't as good for that as it could have been. It has also been reported that the matching between sides is not good enough to trust a common cathode resistor (in fact RCA sais this when it was introduced). [/quote]

I have a couple of 6080 WC from JAN Philips ECG, same date code, september 1971, (that's when I was born, so probably I chose them for a sort of nostalgic feeling )
they have the triodes with very good match, so I think the single R6 resistor is giving good results.

2A3 is a much better audio tube. Don't like current prices?

Click to expand...

No, I don't: each 2A3 has a cost of 33 euros for the Sovtek tubes, and for the amp it would be a cost of 132 euros, compared to a price of 7.75 for each philips 6080, for a total of 15.50.

Keep on posting your comments.

Thanks,


Val.

I saw an OTL 6080/6AS7 amp (I think maybe headphone-oriented as well) in a circa-1970's (?) Japanese amplifier circuit collection paperback that belonged to Rich May.

They are awkward low-mu big bottles. I also saw a audiophool multi-chassis preamp that used them as output tubes, I believe in common-cathode (there's a camp that hates cathode followers).

A (half)Watt of headphone power is large. I use "only" 200mW for monitoring in the same room with loud orchestra. But many headphone amps do have more power.

One thing: you do NOT take the headphones off without muting or unplugging. I used to leave phones plugged into a mixer with a very powerful headphone jack, didn't know how loud they were, and melted a few.

Most of these iPod things supply 15-150mW. And I already see the effect on the young iPod-addicted generation.

If you have an oscilloscope: monitor the voltage on your headphone, any existing amp you have (even iPod). For 32-ohm phones, it is rarely over 1V-2V peak. 300-ohm phones need more, often 5V-10V peak.

I have a theory that 99% of all headphone models (and 99.9% of all headphones made) will be "really plenty loud" if the amp can supply 7Vrms no-load and sags like a 27 ohm resistor. Yes, that does come out to 450mW/32ohms (120mW in 300 ohms). But that's awful loud.

"Power amp" sensitivity for full power is conventionally 1V-2V with no gain knob, 0.1V-0.4V with a knob. Your plan of setting max deck output equal to max speaker/phone output works IF all your signals are at nominal level. But what if you need to crank-up a "silent" passage to check for stray noises? My headphone amp has 0.1V sensitivity. I turn the knob above -20dB (1V sensitivity) only a couple times a year and then only for a few seconds, but it is handy to check for that -90dB buzz which is inaudible on location but will come-up in mastering. This is less important when fed from a DAW; these will usually give "infinite gain" inside the box; not an option on my analog rig.

> Oh, NFB

You put on some resistors. You didn't add effective NFB.

If you did (as by using different resistor ratios), the gain would drop, and I believe the gain is marginal enough now.

As drawn, I don't see a big need for NFB. The amp may run 3%THD at full 472mW, but at ~120dB SPL your ears will run >5%THD. At more normal 1mW-10mW average levels the THD will be low and very simple. May read 1% on the THD meter but won't be as offensive as 0.1% of high-order transistor distortion. Output impedance is plenty low already: maybe 15 ohms. I have not seen a low-Z phone which really needed damping. Some hi-Z phones do need a low-Z source, but that is relative and 50 ohms is plenty low.

> I have a couple of 6080 WC from JAN Philips ECG, same date code,

Input transformer, volt-amp triode, direct coupled to one section 6080 cathode follower. Work the 6080 around 80V and 100mA. Use 1,000uFd electro cap to headphone jack. Yes, it has an electro cap, and yes, it thumps bad at turn-on. But it is easy, simple, and will push most phones plenty loud. You get two channels in one 6080 bottle (and one 6CG7, 12AT7, 6DJ8, etc). Keep the other tube as spare for your old-age, or double-up for a little more lo-Z drive.

Overall: if this is for a DAW Studio, I think it is overkill. Studios are already hot and crowded without some 6080s hanging around. Also: normally what you want (or what I would want) is "flat and accurate". Your simple tube amp will still have some "flavor". Which may fool you into thinking your mix sounds better than it really is. I actually monitor on rather dull consumer amps: if the final listener has anything "good" it should make my mix better than I heard. A very-very-transparent amp is another choice, and many good ears seem to like the ADSL chips as headphone amps. They were developed to push MHz up phone lines with vanishing intermodulation to corrupt the ADSL signal; they loaf passing audio. And it's a cool cigarette box, not a hot bread box. Yeah, the chips are maybe $5, but you may be able to get two as "free engineering samples". TI and National seem to be willing to ship a couple chips to any garage... you could be the next Apple Computer and they want their chip riding a wave like that.

>> Enough to blow your brains out your butt.

> is it possible that would result in some hearing loss?

I have an Asst. Boss, it would probably improve his hearing.

[quote author="PRR"]

Overall: if this is for a DAW Studio, I think it is overkill. Studios are already hot and crowded without some 6080s hanging around. Also: normally what you want (or what I would want) is "flat and accurate". Your simple tube amp will still have some "flavor". Which may fool you into thinking your mix sounds better than it really is. I actually monitor on rather dull consumer amps: if the final listener has anything "good" it should make my mix better than I heard. A very-very-transparent amp is another choice, and many good ears seem to like the ADSL chips as headphone amps. They were developed to push MHz up phone lines with vanishing intermodulation to corrupt the ADSL signal; they loaf passing audio. And it's a cool cigarette box, not a hot bread box. Yeah, the chips are maybe $5, but you may be able to get two as "free engineering samples". TI and National seem to be willing to ship a couple chips to any garage... you could be the next Apple Computer and they want their chip riding a wave like that.
.
[/quote]

oh you're right man, will check that, and hope to be the next "Val" Jobs.

Respect,

Val.

[quote author="PRR"]A very-very-transparent amp is another choice, and many good ears seem to like the ADSL chips as headphone amps. They were developed to push MHz up phone lines with vanishing intermodulation to corrupt the ADSL signal; they loaf passing audio. And it's a cool cigarette box, not a hot bread box. Yeah, the chips are maybe $5, but you may be able to get two as "free engineering samples". TI and National seem to be willing to ship a couple chips to any garage... [/quote]

>ADSL chips< - what chips do you mean?

Had a look at TI, I found just receivers

Rochey has recommended in another thread the TPA6120A2 and OPA1632, which although described as being for audio apparently have some aspects of their roots in ADSL application parts.

Note that they require some careful use of the thermal pad on the bottom to get rid of heat.

[quote author="bcarso"]Rochey has recommended in another thread the TPA6120A2 and OPA1632, which although described as being for audio apparently have some aspects of their roots in ADSL application parts.

Note that they require some careful use of the thermal pad on the bottom to get rid of heat.[/quote]

Oh, TPA6120A2 seems to be the best candidate for this job! Moreover... it is part of the Farnell catalogue in Europe, so this amp should be "Flat and accurate", as many engineers would like.

Did anyone hear about "Balanced cabling for headphones" ?
How to wire in this case?

Look here

[quote author="Val_r"]
Did anyone hear about "Balanced cabling for headphones" ?
How to wire in this case?

Look here[/quote]

No doubt they have to be unidirectional cryogenically treated oxygen free chemically balanced cables! I have some in garage for sale...

While I'm sure we could find better use for that kind of money before we stop laughing I would point out that headphones and headphone amps for that matter typically share a common ground wire for Left and Right ear speaker returns.

Depending upon the wire gauge how and where the two lines are commoned there will be some mutual conduction and heaven forbid crosstalk.

In the good old days when this system of 3 wires for 4 circuits became standard, crosstalk in a good phono cartidge might be 20+dB so no big yuck. These days with digital playback media there actually might be a subtle separation improvement from more discrete headphone wiring.

Or not...

JR

Do you mean $4,000 for bridged headphone amp?


[quote author="mediatechnology"]Wavebourne: Did you look at the prices? $300-400.


Push it a little harder fellas. 472 mW just isn't getting me there...[/quote]

I actually designed a headphone amp back in the '80s at Peavey that drove the common ground leg opposite (L+R) so you could double the voltage swing with opamp friendly rails, and put some decent level into 600 ohm cans.

Looks like they're still selling it, but for less than $4,000.

JR

[quote author="JohnRoberts"]I actually designed a headphone amp back in the '80s at Peavey that drove the common ground leg opposite (L+R) so you could double the voltage swing with opamp friendly rails, and put some decent level into 600 ohm cans.

Looks like they're still selling it, but for less than $4,000.

[/quote]

Loosers? :wink:

I don't know. They're still making and selling a product I designed for them 20 years ago... I suspect the made their tooling back by now... :roll:


JR

Yes, -(L+R) is a brilliant idea! If to LPF it there will be a nice bass boost for low voltage powered devices!

[quote author="mediatechnology"]John - I've been occasionally thinking about that very same thing (minus L+R drive for the common) for about the last year or so. Obviously it's been back-burner.

One additional advantage that L+R drive has in addition to more swing is coupling cap elimination. I know a lot of the headphone driver ICs for single-ended use do provide a DC-driven common to eliminate the cap but I've never seen one that provides signal+DC offset on the common. Your idea is very clever.[/quote]
Coupling caps were not a consideration for me since I was running from +/- 15V rails already. I don't know if I was first or only one, but I did this in mid '80s after being in a recording session and hearing how loud the drummer's headphone monitors were playing... I was working on a line of recording products for AMR (division of Peavey). Some high end headphones were 600 ohms. Most real studios would just use a repurposed 100-200W power amp with appropriate build out resistors for the studio in-wall headphone distribution loop. These days the lower impedance Sonys are popular (more like 30-40 ohms?), so don't need as much swing.

If you look at what's going on it isn't strictly equivalent to bridging both channels so won't give 2x swing for all possible combinations of signals but it worked nicely in practice, and would actually drive a small speaker OK (not your typical wimpy headphone amp). I ended up current limiting the power supply with a resistor in series with the rectifier since playing a speaker too loud would open the thermal fuse in the 1A wall mount transformer. With the power limiting resistor it was "customer proof" even when grossly misused (an important Peavey design criteria).

JR

It's been a few decades since I loooked at this but my recollection is that mono is more like 1x while you do get 2x for pure L or pure R signal. This gives a modest benefit in headroom but not as much as 2x on mono would where the bass energy predominates.

JR

I guess you need a feedback R for the central op amp and then note what the R weighting actually is. EDIT II: Or I see you could just be presuming a fixed gain internal to the middle amp.

Are we averaging or truly summing?

It's a pretty interesting area to consider, and more subtle than it might look to begin with.

EDIT: and you are well to put the copyright---I fully expect to see this or a variant floated in patent purgatory.

Before you boys and girls start building one, it isn't quite that simple.

If you drive the left ear speaker with +(+left) and -(-(L+R)) you will have a R term in the left side.

More appropriate matrixing is +(L-R) and -(-(L+R)). Now you have 2L and no R. Drive the right ear speaker with +(R-L).

If this is patented my (published, as in made and sold) art is some 20 years old so I don't see much chance of a defendable patent. Of course that wouldn't prevent some wet behind the ears examiner from granting one.

JR