Submini Headphone Amp Help

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n00beR

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Joined 2004
#1
Hi all

I have been waiting to take the plunge into tube amps for a few years now. However I, like many newbs, are put off by the voltages involved.

I have experince in designing and building my own opamp circuits.

My specific interest is in sub miniture tubes.

The applications i have in mind are space specific, like on my desk at work.

I would be driving sennheiser HD600's

I have heard good things about the 6111 and 6021 tubes.

Most of the schematics i have found are for guitar amps. I like this one the SubCaster, but again it is guitar specific.

I also like Pete Millett's Starving Student, because of the simplicity, but would like something smaller.

No pleasing some people eh? :ashamed:

I did search through 30 pages of threads but did not find what i was looking for.

Will an all sub mini solution be sufficient for driving the 600's?

Could you guys point me in the right direction or provide me with some simple, low voltage (up to 50v?)schematics.



Thanks in advance

n00b
 
#2
With a 50V supply, I can't really see it happening. The 6111 and 6021 have excellent sonics.......when properly applied. Neither of them will really wake up at 50V. It may be possible transformer coupled in class A2 operation, but there are defiantly simpler approaches that would produce better results. They may be useful for a guitar application with a 50V supply, but not for hi-fi.

You may want to experiment with space charge tubes if you are not comfortable with high voltage. But, in my experience they tend to not be linear enough for Hi-Fi. I'm sure good results can be had, but it would take some determination. None the less, space charge tubes are fun to learn on and experiment with.
A good starting point for space charge tubes.
http://www.duntemann.com/12vtubes/12vtubesindex.htm

You may also want to consider purchasing a kit. It's a good way for the n00b to get a taste, with minimal risk. Just know general electrical safety beforehand. Make sure caps are discharged before servicing. If measurements must be taken with the power on, do so in a manner where you only have to use 1 hand. The other hand should be in your pocket or other place away from the equipment. Don't mess with any circuits that don't use a power transformer(some circuits rectify the mains directly with no transformer in between, this is dangerous).
 
#4
If you mean for low voltage application. If your determined for all-tube low voltage, you may want something along the lines of a 12k5 or 12CX6. You would probably get by with a single stage. But you will be going into uncharted headphone amp territory as far as I know. If you want to do a tube gain stage with solid state-buffer output, there are many possibilities. A lot of the small signal space charge tubes are pretty linear, but they are ones that are incapable of driving the headphones direct.

As far as tubes for regular tube voltages go:
6080 or 6as7 based designs are pretty popular and for good reason. They have a low plate resistance which makes them able to drive the headphones directly with ease. But your still going to need a 80-150V supply. Run a google search on 6as7 or 6080 headphone amplifier, you will see plenty of projects. I think some dude on ebay even sells kits.

A subminiature headphone amp could work well, but it would have to be a transformer coupled amplifier(to better match the impedance to the load) operating at around 100V-150V.
 
#7
There was some low voltage tube stuff described on http://www.tubecad.com/ but I can't recall if it was suitable for headphones or not. Lots of interesting information there, but might take you a couple of weeks to read through it...
Click to expand...

If my memory serves me well, he was using the ECC86 in all his low voltage designs. This wouldn't make a good candidate for a low voltage headphone amp because it runs at low current (2mA I think). He did have a design for an Aikido using 24V and ECC86, with a transistor buffer for headphone outputs ( I actually think he used LM317's as the buffer stage !!).
The other disadvantage is that since he mentioned them they have experienced a price spike and availability crisis.

Shoog
 
#9
Hello again

After some further readng I have been looking at pocket radio tubes, would the 6088 tube be a better tube for my amp?

If transformers were needed, what sort of ratio would i be looking for?

Thanks in advance

n00b
 
#10
The 6088 looks like an interesting candidate, but it falls short in a number of ways.
Distortion will be high - 10% at max output.
The plate current is rated at 650uA which is still only 0.65mA which is worse than the ECC86.

Think about one of the dissimilar dual triode valves.

Look at the PCL83. This is a triode pentode audio valve. Unlike its brothers the ECL82 and ECL86, it seems to have its triode mu of just 14. In using the ECL82 I have always found the gain of 60x of the triode section to be a huge pain, the PCL83 avoids this. A nice little parafeed amp could easily be built using power toroidals and plate load CCS's. The voltage would have to be about 250V though.

Shoog
 
#13
n00beR said:
Hi all

I have been waiting to take the plunge into tube amps for a few years now. However I, like many newbs, are put off by the voltages involved.

I have experince in designing and building my own opamp circuits.

My specific interest is in sub miniture tubes.

The applications i have in mind are space specific, like on my desk at work.

I would be driving sennheiser HD600's

I have heard good things about the 6111 and 6021 tubes.

Most of the schematics i have found are for guitar amps. I like this one the SubCaster, but again it is guitar specific.

I also like Pete Millett's Starving Student, because of the simplicity, but would like something smaller.

No pleasing some people eh? :ashamed:

I did search through 30 pages of threads but did not find what i was looking for.

Will an all sub mini solution be sufficient for driving the 600's?

Could you guys point me in the right direction or provide me with some simple, low voltage (up to 50v?)schematics.



Thanks in advance

n00b
Click to expand...

So long as you always switch off and put the plug on your bench and then discharge the high voltage caps through a resistor until the volts are negligible then you should be OK.

Dont do what I once did and just short out the cap ! there was a massive flash and bang and I nearly fell of my chair !

But you do have to be ultra careful with valve amps.
 
#17
Some of sijosae's circuits are kinda' silly, but they often sound surprisingly good in spite of all that sillyness.

Back on to the OP's question:
For one of my circuits, I use a 160v B+ on a 6111wa w/ a 100k plate load, a 2.2k cathode resistor, 470uF bypass cap, 100ohm grid suppressor (probably not necessary), capacitor-coupled to a DC coupled Szekeres style buffer, but running at around 100mA of idle current. The whole cicuit (minus the psu) is inside an altoids tin, w/ the tube sticking out the top. Whole thing runs pretty warm, and sounds quite nice. For other simple designs, try a google search on yaha amplifier. that's the basic gain stage, just add a cathode resistor(bypassed or not, your choice), and either keep the opamp buffer, or use something else. I personally don't care for opamps, so I often use szekeres style power stages. If you don't mind fooling around and experimenting, try something along the lines of a low-voltage design w/ 6l6, el84 or similar. Those are my next projects. 😉
 
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