Lampy cd player

Post by **Nick** » Fri Mar 25, 2011 9:57 am

Yep, anode of top valve is good. What I am checking is how the output stage reacts to a injection of current into it instead of out of it. Under AC conditions with a capacitor coupling to the load, current will flow in both directions, so this lets you check how symmetric the output stage is. People get trapped into thinking the signal "flows" from the output, but other than the vector product in MWE, its symmetric.

Richard · Post by **Richard** » Fri Mar 25, 2011 1:59 pm

Dang! I kept refreshing the page but no reply then saw we're on a new page. Anyway 2 cups of tea and a bowl of soup later we have the figs. Thanks for explaining, I was looking for relevant B+ ac and dc paths and couldn't see what we were doing.

Interesting results anyway not least in that the two valves don't have the same voltages on them when sitting quiescent dc as might be assumed. Glad too that I put in a heater raise of 34V. Not sure what the g2 fig tells us, it's definitely there and seems constant. The input is connected to the 1541 analogue op and that 100 IV R is 82R in mine.

(I did some 100K figs too and they fall between these 50K and open op figs; with 100K to ground, a2 was 69V etc)

In the circ below I've taken the top triode as 1 and the bottom as 2.

Open op;

a1 174V
g1 97.1V
k1 97.4V
a2 97.1V
g2 0.174V
k2 0.253V bottom cathode (= 1.26mA)
across top cathode 0.253 (= 1.26mA)
drop on last 10K dropper in B+ line 13V (=1.3mA)

op>50K>ground

a1 163V
g1 51.1V
k1 51.4V
a2 51.1V
g2 0.175V
k2 0.131V bottom cathode (= 0.65mA)
across top cathode 0.351V (= 1.75mA)
drop on last 10K dropper in B+ line 18V (=1.8mA)

op>50K>B+(a1)

a1 165V (there is this 2V difference possibly just the slightly less load on B+)
g1 124.1V
k1 124.3V
a2 124.1V
g2 0.175V
k2 0.326V bottom cathode (= 1.63mA)
across top cathode 0.164V (= 0.82mA)
drop on last 10K dropper in B+ line 17V (=1.7mA)

Post by **Nick** » Fri Mar 25, 2011 2:55 pm

Well, from my rough interpretation of the numbers, I think the load to 0v looks like a source impedance in the order of 92k, and from load to b+ 67k.

Both in the area a would expect and being the result of the ra of the lower valve.

To me that shows that the upper valve is not functioning as a cathode follower.

It would be interesting to repeat the tests but taking the load from above the upper cathode resistor and see what the output impedance is from there.

Mike H · Post by **Mike H** » Fri Mar 25, 2011 4:34 pm

Groping in the dark here but summat smacks of not being quite right.

In "open op", A2 is 97V, I should have expected it to be almost exactly half supply, assuming A. its 2 halves of the same double-triode; B. they're well matched.

I.e. if B+ were 150 (as written on diag) then I would expect A2 to be 75, actually 75 across each triode if they're biased the same, which they are.

op>50k>ground; op>50k>B+; is that by just connecting 50k, there's no DC blocking capacitor in series?

Richard · Post by **Richard** » Fri Mar 25, 2011 9:34 pm

Hi Nick,
So around 38K, half typical common cathode anode output for this valve. Should get away with it on short i/c's but will check f response with a test cd into 50K across the scope. Will it be linear? Should I run it or give up on it now?

Hi Mike,
Had my curry, onto the cote du rhone, and into the last tests of the day

This was meant to be playing music tonight

The results are indeed for sections within a single twin triode valve. They're new from Russia so should be ok

but I did buy 6, checked them on the AVO, all very consistent, and paired them up as well. The results are good if not outstanding, and confirm the "not equal voltages" as noticed before.

The cathode resistors are all the same 200R and when run open output paired sections must be passing the same current but does that mean the voltages have to be the same or is it due to one valve having higher impedance than the other perhaps, as Nick's calcs?

L channel

a1 174.8V
a2 97.9V
total current 1.29mA

R channel

a1 174.4V
a2 92.8V
total current 1.33mA

Mike H · Post by **Mike H** » Fri Mar 25, 2011 11:04 pm

A2 still seems a mite high, by about 10V. Not sure what's going on there, pernickety as I may be

2nd observation ~ the upper triode(s) has no grid stopper?

Post by **Nick** » Fri Mar 25, 2011 11:22 pm

They are within 20%, probably as good as you can expect, normally you hope that they are matched with respect to gain, gm is normally less of a worry. I wouldn't have though a grid stopper was needed, its effectively a ecc83

Mike H · Post by **Mike H** » Sat Mar 26, 2011 12:03 pm

Fairy nuff!

Richard · Post by **Richard** » Sun Mar 27, 2011 1:55 pm

Hi all, up and running and sounds pretty good but tbh I preferred the exact same version but using 2604 opamps instead of this valve stage. Sound is bigger with the valves, huge in fact, and harmonics abound but focus and sparkle iis missing and it doesn't sound as clean.

So this morning thought I'd check f response. The pics below are with it connected to the scope with my regular i/c terminated with 47K (into the scope's 1M). Throughout all this I also have set up an identical totally stock cdp. On listening tests it was getting left behind until this valve stage where now it sounds boring butt somehow more correct.

I also checked the stock player using the same test disc, i/c, termination and scope and the spot sine waves are as clean as a whistle.

I checked on spot frequncies 2Hz to 20KHz. Level is flat after 8Hz and op voltage a little higher than stock.

So these below are at 200Hz, 1KHz, 10KHz and 20KHz.
The level stays the same which is good but there's clearly something wrong.
I'm tempted to think it's the valve stage. The mods so far are 2604, coupling caps, NOS in that order and all sounded good til now but I didn't scope the NOS.

Post by **Nick** » Sun Mar 27, 2011 6:41 pm

Is that a full scale signal being displayed?

What is the voltage from the DAC? How much gain do you need?

I have to admit, if I was asked to suggest a output stage for a DAC, a ECC83 SRPP would not be the first thing that sprang to mind.

Richard · Post by **Richard** » Sun Mar 27, 2011 11:08 pm

Hi Nick, yes that's 0dB out of the player displayed at 2V/div so around 3V compared with the standard player which shows fine at 2V.
Yes the valve doesn't look ideal. I did the figs for an srpp into 47K and got an output impedance of 34K. Not that the frequency response looks bad, level looks flat. Gain would be too much with an srpp ecc83 proper though. It looks like he's lost gain with this hybrid sort of circ but if this is the result (and not my mistake) then it dsoesn't look good. So a quick change to Thorsten's ecc88 will be next up I think.

Richard · Post by **Richard** » Sun Mar 27, 2011 11:22 pm

Re dac op level etc I did some research but haven't yet checked anything myself. Idea was to get up and running then see if it needed fixing rather than go over known stuff. I was going by this,

The TDA1541A current outputs are constructed different compared to the TDA1543. The TDA1543 allows / tolerates large DC / AC voltages at the current outputs. The TDA1541A does not, here, both DC and ac at the DAC outputs must remain low in order to keep distortion low. The TDA1541A ac output signal will clip at approx. 900mVpp.

When passive I/V resistor value is too low, S/N ratio is poor. So it's best to use max. possible output amplitude while keeping distortion low.

Practical passive I/V resistor values for the TDA1541A vary between 82 Ohm (328mVpp) and 100 Ohm (400mVpp). The I/V resistor is connected between TDA1541A output and AGND (pin 5).

Next, some amplification is required to achieve desired output amplitude.

In order to determine the best performing analogue circuit, the I2S signals must be virtually perfect (extreme low jitter amplitude, flat jitter frequency spectrum, and lowest possible I2S signal crosstalk). With the SD-card player and 12-crystal lithium-cell referenced precision master clock, the passive I/V conversion appears to works best for the TDA1543. The TDA1541A appears to perform best with the trans-impedance converter. Only big problem is achieving lowest possible power supply noise for the trans-impedance converter.

highlighted in a very long thread,
http://webcache.googleusercontent.com/s ... ogle.co.uk

Neal · Post by **Neal** » Mon Mar 28, 2011 9:25 am

Richard, are those traces made using the TDA output or where you injecting the test signal?

They look very similar to the output of a my TDA1541 (NOS) based Arcam DAC I messed about with last year...the anti-aliasing filter is ineffective using NOS. I made a small improvement to the output by messing about with the TI filter designer but the signal above 8Khz was still not clean.

Richard · Post by **Richard** » Mon Mar 28, 2011 12:03 pm

Hi Neal,

They're spot frequencies at 0dB played from a test cd through the TDA1541 set for NOS cdp and the valve output stage.

I didn't scope the effect of doing the NOS mod (darn!) so they could indeed be due to either the NOS mod or this valve op stage.

So, which should I blame? I suppose, rather than pull the valve stage to pieces just yet it wouldn't be hard to remove the NOS mod (reinstate 4x OS)and scope it again to see if its cleaned up.

What would you do?

Post by **Nick** » Mon Mar 28, 2011 1:22 pm

As long as you don't look at them on a scope, NOS 1541 without a reconstruction filter should sound good. You may prefer others, or with the OS and or reconstriction filtering. But IMHO the sound you are describing is not down to the DAC.

I played other games with it (driving a pair of 1541 differentially via a CPLD) but if you have them on hand, try cap coupling the I/V resistor via a set of 1:10 MC step up transformers to give you voltage gain. You may be supprised how it sounds. (just to get a sense of what the DAC is doing).

audio-talk

Lampy cd player

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