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Tube-I-zator Professional PCB

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C

Crom

Member
Joined 2012
DQ828 said:
DVD
Do you have any idea what value would be suitable for BIII as the I/V resistors?
Click to expand...

Can anyone explain to me how best to work out the best value for the I/V resistor? I've had a play with a few values ranging from 20R to the original 56R and I have to admit that I'm not sure what I'm listening for...and I can't tell what, if anything, changes. Does anyone know what I should be listening for...distortion? changes in output volume etc etc?

Thanks,
Crom
 
N

noizas

Member
Joined 2009
Crom said:
Can anyone explain to me how best to work out the best value for the I/V resistor? I've had a play with a few values ranging from 20R to the original 56R and I have to admit that I'm not sure what I'm listening for...and I can't tell what, if anything, changes. Does anyone know what I should be listening for...distortion? changes in output volume etc etc?

Thanks,
Crom
Click to expand...

Hi Crom,

sadly to say, but both I/V (more correct name should be I/U) resistor values 20R or 56R compromise TDA1541 linearity and resolution.
However to detect those problems you need perfect set of your ears, loudspeakers and amplifiers :), else you need to measure THD somehow.

Value of I/U resistor is possible to calculate from TDA1541A datasheet:
1. "Notes to the characteristics: To ensure no performance losses, permitted output voltage compliance is ±25 mV maximum." - here is P-P values, not RMS.
2. "full scale current: 4 mA (nominal)" - current is negative here.
3. ""zero scale current: 25nA (nominal)"- actually 0 mA.

So there is two ways to calculate maximum correct value of I/U resistor by Ohm law R=U/I :

A. If you have no constant 2mA current injection circuit -
R=[-25mV]/4mA=6.25R :eek:, because current is only negative here.

B. If you have constant 2mA current injection circuit -
R=[+-25mV or 50mV P-P]/4mA=12.5R :rolleyes:, because current is swinging +-2mA around 0 (compensated by CCS).

So, why most of diyers use I/U resistor for TDA1541 in range 20R - 60R ?

It is because of noise issue as well as amplification factor.

Lets take circuit with CCS 2mA injection, so we have signal of U RMS=0.7*25mV=17.5mV, and we need to amplify it to:
1. For CD player output level (2V RMS): K=2000mV/17.5mV - about 114 (even more than 12AX7 in grounded cathode way can provide, not even mention SRPP - half of mu).
2. For level of professional audio (+4dBu, or 1.23V RMS): K=70 (still not achievable with SRPP and 12AX7 tube, but almost here ;))
3. For level of customer audio (-10 dBV or 316 mV RMS): K=18 (easily achievable in SRPP with 12AX7 and almost achievable with medium mu tubes as 6N1P or 6DJ8, etc).

But choosing tubes with medium to high mu, do not forget standing current in SRPP - more standing mA you have, less the output will be taxed for charging capacities of interconnect cables and input of preamplifier.
This is also related with output impedance - as usual medium mu triodes have less plate resistance than high mu triodes, so damping factor of interconnection would be better.

Best wishes
Saulius
 
LucasAdamson

LucasAdamson

Member
Joined 2009
As I understand it, the circuit is one tube per channel, so both tubes need the resistors. They are to convert the 150v of the Salas HV regulator to 2mA for the TDA1541A, to provide a stable and smooth CCS for obtaining the correct output level.

So, for 2xTDA1541A, you'll need 2 resistors per channel (4x resistors in total), but for a single TDA1541A, you'll need one resistor per channel, so order 2x resistors.
 
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dvb projekt

dvb projekt

Member
Joined 2009
Paid Member
LucasAdamson said:
As I understand it, the circuit is one tube per channel, so both tubes need the resistors. They are to convert the 150v of the Salas HV regulator to 2mA for the TDA1541A, to provide a stable and smooth CCS for obtaining the correct output level.

So, for 2xTDA1541A, you'll need 2 resistors per channel (4x resistors in total), but for a single TDA1541A, you'll need one resistor per channel, so order 2x resistors.
Click to expand...

Correct! :up:
 
C

Crom

Member
Joined 2012
noizas said:
Hi Crom,

sadly to say, but both I/V (more correct name should be I/U) resistor values 20R or 56R compromise TDA1541 linearity and resolution.
However to detect those problems you need perfect set of your ears, loudspeakers and amplifiers :), else you need to measure THD somehow.
Click to expand...

Saulius, thank you very much for taking the time to provide such a detailed response. I clearly have much reading and learning to do ;-)

I am using the Buffalo IIIse (rather than the TDA1541A) but I think I understand the logic. I'll see if I can find the datasheet for the Sabre 9018 and calculate the values.

In case others are interested there's some relevant stuff on the TP site: Questions on es9018 outputs - Buffalo DAC - Twisted Pear Audio Support

Cheers again,
Crom
 
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