I need help with ccs based LTP

[pauldune]

Just did some measurements:

Gain: 35x SE (so that is +/- 70x with a differential input)-> nice!
Distortion: ~ 0.01%-0.02%; see FFT attached-> really nice! I had some troubles with the EMU 0202; output noise was a little to high, but in the end i'm not disappointed.

Things to do:
-Measure distortion with the differential dac connected to the input, and a test CD.
-Build a DC heater.
-Redesign PCB.

The schematic is as attached; component values:
R1=R2=100 Ohm, 1/4W, metalfilm 1%
R3= adjustable 100 Ohm; now set for 48 ohm. adjust it so Vanode/plate is 160v (beware I made a mistake in the drawing, pin 1 of lower ccs should be connected under R3)
I used 1Kohm gridstoppers on tubes and CCS's.
R4=R5=100Kohm, 1W, metalfilm 1%

[Kay Pirinha]

Just a thought, maybe a silly one, I've never tried it though: What about a pair of hfe/vbe-selected video pnp transistors, connected as current mirrors, as plate loads?

Best regards!

[pauldune]

Hi Kay,

What is a current mirror? Same as a CCS?

I tried the schematic with the 3 ccs's directly connected to the differential current output from a DAC, and it works perfect! Direct connected; so no caps on the input; just one on the output. Distortion is the same as above; will upload the screenshots this evening.

Paul

[pauldune]

The distortion figures with DAC input; i/v conversion 2x4.7 ohms (differential)
0db 16bit 44khz samplerate 1Khz sinus input; Vrms out max: 1.2V

[Kay Pirinha]

Current mirror from Wikipedia...

Best regards!

[pauldune]

Originally Posted by Kay Pirinha

Current mirror from Wikipedia...

Best regards!

Ok, i get it, but what do you want them to mirror? Not the current from the other branch, i would think, because thats normally exactly the opposite.
Could you draw a simple concept? And explain what you want to accomplish?
(if you want)

Paul

[7N7]

Interesting thread.

In 2003 I was planning a rather elaborate amplifier and had prototyped most of it. I fancied the idea of active loads and CCS and developed the stage despite a number of folk telling me that the idea wouldn't work. See Sy's post on the first page of this thread though - I used MJE350 for the active load and the CCS was a typical Morgan Jones cascode, but without negative supply (I can't remember why), Of course the pentode loads were CCS as well. It used a 7N7 as the gain part and this was followed with a pair of E810F pentodes arranged as (pentode) cathode followers; The pentode cathode followers were intended to be DC-coupled to the output stage and therefore sat on a -280V rail.

At the time I had a reasonable collection of test gear and was able to measure distortion up to sixth harmonic, The complete stage delivered 77.5V rms into a 30k load with distortion more or less at the floor of the test gear - -64dB = 0.063%; amazingly third harmonic was at -87dB,

Of course I never used it since my circumstances changed rather radically. I still have a circuit diagram scribbled,

Paul

[pauldune]

Hi Paul,
Sad to hear you couldnt use your design because of circumstances. Sometimes life doesnt play nice...

In case some of the readers are interested, i'll attach the complete schematic. I realize I haven't done that before. It is stereo, because i'm designing a stereo pcb in eagle.

Paul

[grufti]

4R7 on the input?

[pauldune]

Originally Posted by grufti

4R7 on the input?

Yes!
The input is a 16mA current output from a buffalo 3 dac.
Essentially the 4.7 ohm is the current to voltage converter. The buffalo needs an impedance as low as possible to get the best specs.
4.7 was the calculated value to give a maximum output of aprox. 2v out of the tube stage. Because the gain now with all the ccs's is higher as with a normal ltp, I get aprox. 3 v out. Oh and the coupling output cap isnt on the schematic because it isnt on the pcb. I have a russian teflon and a russian silvermica, i want to try wich sounds best. But both are too large to fit on the pcb.

Paul