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Tube amp for both AKG K340 and Grado (µ-follower) - Page 2

post #16 of 24
10/13/10 at 8:57am

 

Quote:
Originally Posted by regal View Post

This is a mystery to me as well,   I the 100 ohm resistor is incorporated with the voice coil electrically so in essence you are increasing the damping factor.  Its sort of what came first the chicken or the egg argument.   Of course I have been wrong before.


You are reducing the damping factor. 

 

The headphones see a source impedance of 100ohms VS ~0.

 

The amp sees 132ohms rather than 32. I think its key that this test is done with a big uberpower SS amp to eliminate/minimize the variable of the amp distorting differently under different loads. 

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post #17 of 24
10/13/10 at 9:34am


 

Quote:
Originally Posted by nikongod View Post

 


I heard that amp at canjam. It was quite nice.

 

I dont think its output impedance that screws up headphones. I think its how the amp distorts.
 

Quote:
Originally Posted by regal View Post
This is a mystery to me as well,

 

I agree with Ari on this -- I actually find that Grados do best with a fairly high Z out, but cathode followers don't do well with the low load, so it does not work well.

 

A very cheap version of the Caged Frog can be built by subbing a DN2540 for the pentode, using a LM317 as the power supply (24V works just fine) and a $40 pair of cheapo Edcor OPTs. The whole thing will cost you $60. Not the most detailed amp ever, but one with a similar enough sound signature to get a taste of the Gm amp. I use one all the time -- run at ~60mA I think, and with a 8R dummy load on the 5K:8 transformers.

 

As for Ben's original idea, I don't necessarilly mind the OTL nature of it -- it's basically a form of parafeed which I find more appropriate for phones than single feed. But, you might look into a Beta follower -- I've not seen anyone drive headphones off one, but it looks like a promising idea.

 

Quote:
Originally Posted by regal View Post
Of course I have been wrong before.

 

Indeed smily_headphones1.gif

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post #18 of 24
10/13/10 at 10:21am
Thread Starter 

 

 

Quote:

Originally Posted by dsavitsk View Post

 

But, you might look into a Beta follower -- I've not seen anyone drive headphones off one, but it looks like a promising idea.

 

 

The big advantage of the mu-follower is the fact that we can use a big tube atop a small one, using their respective strength. A beta follower must use two big tubes.

 

But I'll agree that it would still be worth investigating with a proper srpp and other tubes.


Edited by 00940 - 10/13/10 at 10:25am
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post #19 of 24
10/14/10 at 2:53am
Quote:
Originally Posted by dsavitsk View Post
 But, you might look into a Beta follower -- I've not seen anyone drive headphones off one, but it looks like a promising idea.


I spent a long time with a (direct coupled) beta follower before going back to a super-mu stage - CCS load, lower anode cap coupled to the cathode follower grid. Running just over 20mA is the sweet spot for a 6H30 (gain triode) and 6C45 (cathode follower).

 

super_mu.jpg

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post #20 of 24
10/27/10 at 6:54am
Thread Starter 

While waiting for parts to arrive, I've been back to the drawing board. Here is what I'm finally going to build:

 

- I keep the transformer-coupled cathode follower theme but switch from SE to push-pull, to get rid of the parafeed cap.

- The input stage needs now a fair bit of gain, so I switch from the ecl85 to the ecl84, which has a high-µ triode. With the xformers I've got, the total gain (input stage gain/transformer ratio) should be around 5.
- I need to provide for a huge, linear voltage swing in one stage, so I use both a CCS in the tail of the input LTP and gyrators on the anodes (idea from Michael Koster: http://www.diyaudio.com/forums/tubes-valves/162799-what-tube-differential-amplifier-2.html#post2114387 ).

- The output stage iddles at 25ma (12ma per tube) and the input stage at 3-4ma.

 

The ever optimistic LTspice promises 50Vpp into 300R before reaching 1% thd, without any global feedback. I've fooled around with values to see what would happen with unmatched parts or tubes and the topology is not very sensitive to small changes. It's also quite tolerant of its power supply, a crcrc supply should be sufficient but I'll use a capacitance multiplier, in order to get a soft-start of the B+ rail.

 

Sans titre.gif

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post #21 of 24
10/27/10 at 7:58am

Neat circuit!

 

I think that the first stage has CCS's in series. I would keep an eye out for trouble when you build it.

 

I posted a similar question to audioasylum a while ago and got the "folding" answer here:

LINK

 

Im not sure if anyone has built the circuit you linked to, and I have not modified the circuit I posted my question for. Try it, but be prepared for the thing to be barely stable (or to lock onto B+/ground for no apparent reason). If it happens it sounds like a high value resistor from each plate to ground should stabilize it.


Edited by nikongod - 10/27/10 at 8:57am
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post #22 of 24
10/27/10 at 4:50pm

how high are the ecl84 cathodes above ground(the outputs)?


Edited by digger945 - 10/27/10 at 5:04pm
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post #23 of 24
10/27/10 at 5:18pm
Thread Starter 

Something like 150V at idle. Maybe 220V on strong peaks. The cathode/heater voltage limit is 200V on those tubes, so it'd be good to raise the heaters to 75V or maybe even up to 100V.

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post #24 of 24
11/17/10 at 12:29pm
Thread Starter 

Small update.

 

I've more real world work right now than I thought I would and thus the project is a tad delayed. Probably will be done during the Christmas vacations. I might build the PS before.

 

Still, I've decided to rework the schematic a bit, for the sake of simplicity. The gyrators in the ltp will be replaced by a simple resistive load for a start. I can always add it later if I feel it is necessary. Tube matching bothered me a bit. In the ltp, it is no big deal thanks to the CCS, but it could be a problem for the output tubes. If the tubes are too mismatched, the two sides of the xformer won't be balanced. So I will add a 10K trimmer to be able to slightly adjust the grid voltage of one of the output tubes. Lacking on the schematic are two 10r resistors in the cathode of the output tubes, to monitor current draw.

 

I added the PS to the schematic. It's a very basic reg but it allows for a not too fast start-up. All caps motor run MKP, 20uf/450VAC. I'm using a xformer I had. It's a 330-0-330, which should bring 460VDC. A bit high obviously. I'm burning 60V through big zeners (which also gives me the negative supply I need for the ltp ccs) and the last 50V will be for the mosfet (an IRFBC40, 600V part). The amp should burn around 60ma (for the two channels), the mosfet won't have to stand more than 3W. Heaters must be raised to 100VDC.

 

Sans titre.gif

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