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Old 12th March 2013, 12:09 PM   #761
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Quote:
Originally Posted by DQ828 View Post
Trying to buy Carbon resistors to meet the requirements (without going to a special OS supplier for 1 resistor) is impossible, can I just use metal film, why the carbon?
It is a recommendation from Salas to use 220R carbon resistors as gate-stoppers.

If you have nothing at hand, use a metal film.
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Old 14th March 2013, 11:56 AM   #762
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Quote:
Originally Posted by dvb-projekt View Post
It is a recommendation from Salas to use 220R carbon resistors as gate-stoppers.

If you have nothing at hand, use a metal film.
Gate-stoppers???

I'm talking about the 820R 2W resistors you recommended I use as a replacement 1kO to raise the input voltage. The 1KO are carbon.
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Old 14th March 2013, 01:34 PM   #763
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Quote:
Originally Posted by DQ828 View Post
Gate-stoppers???

I'm talking about the 820R 2W resistors you recommended I use as a replacement 1kO to raise the input voltage. The 1KO are carbon.
Ups... A small misunderstanding!

You could use any 2W resistor you have for this position!


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Old 18th March 2013, 06:41 PM   #764
Crom is offline Crom  United Kingdom
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Quote:
Originally Posted by DQ828 View Post
DVD
Do you have any idea what value would be suitable for BIII as the I/V resistors?
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
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Old 20th March 2013, 07:55 AM   #765
noizas is offline noizas  Lithuania
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Quote:
Originally Posted by Crom View Post
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
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 , because current is only negative here.

B. If you have constant 2mA current injection circuit -
R=[+-25mV or 50mV P-P]/4mA=12.5R , 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
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Old 20th March 2013, 10:18 PM   #766
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Exclamation Tube-I-zator / TDA1541A recommended modification

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Old 20th March 2013, 10:21 PM   #767
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Hi Oliver,

Type and/or brand of 75K resistor?

Thanks
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Old 20th March 2013, 10:31 PM   #768
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Quote:
Originally Posted by crobbins5421 View Post
Hi Oliver,

Type and/or brand of 75K resistor?

Thanks
Blog entry is updated.

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Old 20th March 2013, 11:04 PM   #769
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Perfect. Thanks.
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Old 21st March 2013, 12:43 AM   #770
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Does this apply to the Buffalo DAC's as well?
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