DC coupled line preamp kit

[ginetto61]

Hi, i was reading a coupling capacitors shoot-out ... and i was getting mad
The prices of the good ones are unbelievably high. Like 100 USD/each
And of course the differences in sound with the cheap ones are huge.
Moreover, after reading about the high impact of different caps on sound i wonder now if an AC coupled line preamp has any chance to be truly transparent.
Therefore I am asking if anyone is aware of kits for a line preamp DC coupled.
Even op-amp based ones will be fine (no prejudices about opamps here).
Thanks a lot and kind regards, gino

 

[knutn]

A good 10 uF Polypropylene capacitor you can by for about 4 Euros, it is not that expensive.

 

[ginetto61]

A good 10 uF Polypropylene capacitor you can by for about 4 Euros, it is not that expensive.

Hi, thanks for the reply and that is what i thought ... prior to reading the shoot-out comparison.
From what i read a minimum to get good sound is something like Mundorf Mcap Supreme, and that is not cheap.
And this was at xover level, where signal is more robust than at line level.
Therefore if there is a kit around that can do without coupling caps that would be very nice.
Maybe the writer was exaggerating the differences, but still i am umcomfortable now.
And i see that very best solid state line preamps come all dc coupled ... there must be a reason i guess.
Thanks again and kind regards, gino

P.S. i hate valves

 

[richie00boy]

There is a big difference in the impact of a capacitor used as a filter in the normal audio passband, to one used for AC coupling and out of the normal audio passband. You do not need crazy caps for AC coupling, don't believe the hype and often non-blind-tested claims!

 

[ginetto61]

There is a big difference in the impact of a capacitor used as a filter in the normal audio passband, to one used for AC coupling and out of the normal audio passband.
You do not need crazy caps for AC coupling, don't believe the hype and often non-blind-tested claims!

Thanks for the reply and please let me elaborate a little.
When i see the number of comments on the Blowtorch preamp i guess that this preamp is really something of special, with a very fine sound, and provokes an immense interest about its design/construction.
Given that this is a very good preamp indeed, i see that luckily is solid state (no more tubes ever in my system) and dc coupled (i assume this; if i am wrong i would like to know which coupling caps are used).
So no signal caps is the best. That is clear.
Having to use caps the question is which are the most transparent around.
I have already an ac coupled preamp.
The former owner told me that he has replaced the original caps with better ones and with the result of a remakable improvement in the sound.
We say in Italy that appetite comes eating ...
Personally i would be much more comfortable with a Blowtorch preamp ... if it were not above my current budget.
But also a kit with a dc offset nulling circuit would be just fine ... just to try.
Thanks a lot and kind regards, gino

 

[jerluwoo]

Also think about the fact that a huge majority of commercial amps have a cheap electrolytic at the input anyway, what is there to gain by using some wildly expensive capacitors for the preamp.

 

[ginetto61]

Also think about the fact that a huge majority of commercial amps have a cheap electrolytic at the input anyway, what is there to gain by using some wildly expensive capacitors for the preamp.

Hi actually you are very right and i agree completely
So ANY let's say decent PP would do just fine ?
or there are some brands with a very nice quality/price ratio ?
I need four 4.7 uF nice caps ... preferably not very big
Thanks and regards, gino

P.S. i have also found something

http://www.mil-media.com/preamps.html

Ultimately, carefully selected capacitive coupling was found most accurate for both front end and interstage coupling.

So this careful selection looks the key to satisfaction ...

 

[DF96]

And of course the differences in sound with the cheap ones are huge.

Are they?

 

[dmills]

Indeed, audiability of a line level coupling cap (if it is even remotely suitable for the purpose) would be something of a surprise to me, filter caps are of course a different thing....

Lots of assertions about caps out there, very little good data (Self and Cordell have both done some work, but it is more applicable to filters then coupling IMHO).

Regards, Dan.

 

[dadod]

Hi, i was reading a coupling capacitors shoot-out ... and i was getting mad
The prices of the good ones are unbelievably high. Like 100 USD/each
And of course the differences in sound with the cheap ones are huge.
Moreover, after reading about the high impact of different caps on sound i wonder now if an AC coupled line preamp has any chance to be truly transparent.
Therefore I am asking if anyone is aware of kits for a line preamp DC coupled.
Even op-amp based ones will be fine (no prejudices about opamps here).
Thanks a lot and kind regards, gino

Maybe you can try this one? http://www.diyaudio.com/forums/solid-state/235695-no-nfb-line-amp-gainwire-mk2-34.html#post3831730

 

[Fast Eddie D]

There is a big difference in the impact of a capacitor used as a filter in the normal audio passband, to one used for AC coupling and out of the normal audio passband. You do not need crazy caps for AC coupling, don't believe the hype and often non-blind-tested claims!

This is the reality right here. For a capacitor to introduce distortion, it has to have a voltage across it. So if you select a coupling capacitor that has negligible impedance compared to the circuit it's driving, it will introduce virtually no distortion. So a 1 uF coupling capacitor for a 100 Kohm input impedance will have a negligible effect on distortion; its turnover frequency will be 1.59 Hz. This assures negligible effect at frequencies below 15.9 Hz. An electrolytic that has no resonances below 200 KHz will be for all practical purposes transparent.

In a filter, the capacitor has a voltage across it at a crucial frequency, and it will introduce distortion. This is where careful selection of a capacitor is paramount, and this is where uber expensive capacitors can be justified. Electrolytics are a terrible choice here (although commercial grade equipment uses them in this application), and metallized polyester film is a good choice if available in a useful value.

Cyril Batemann authored the definitive work on this topic.

 

[Fast Eddie D]

Also, the trend towards using oversize coupling capacitors does not improve the performance of the circuit vs a correctly chosen capacitor. In fact, it can have deleterious effects:

1) Larger electrolytics typically have lower resonant frequencies, which can impinge on high frequency performance in extreme cases.

2) Larger coupling capacitors can cause turn-on and turn-off thumps.

It is prudent to use your input capacitor to set your dominant low frequency pole. 2 Hz and under is your goal. Other low frequency poles (most commonly feedback DC blocking capacitor) should be significantly lower than this; 0.2 Hz is ideal but often impractical. The thing to avoid is "stacking" the poles all at the same frequency.

Prudent design limits bandwidth. Ideal frequency response (3 dB down) is 0.2 to 200 KHz. This provides a frequency response of 20-20,000 Hz +/- 0.1 dB. Frequency response with typical commercial designs is 1.59 to 159 KHz. This should be accomplished at the input.

I know some of you are haters, but this is how commercial equipment is designed; and there are very good reasons for doing it that way.

 

[knutn]

I fully agree with Fast Eddie. And another point, often overlooked, is that for a symmetrical amplifier, although DC-coupled, the signal passes through the power supply capacitors. And what is the quality of these?

 

[Jonathan Bright]

Hi ginetto61,
I am just curious but why do you have such a strong reaction against tubes/valves...........? A bad experience with them? By the way it should not be too hard to make your own DC pre-amp with op-amps that you can dial out the DC offset if needed etc.
Have you tried Googling "DC audio pre-amp"; might be worth while.
Cheers,
Jonathan

 

[jerluwoo]

Aerovox AFPS are a good price/quality ratio cap. Replacing the generic metal films I got from Radioshack in my 1626SE made a very noticeable difference. Axial Leaded Film Capacitors | Aerovox

 

[ginetto61]

Dear All, thanks a lot for the very valuable input
Unfortunately my boss has given me some job to do urgently
Nevertheless i promise to study all your replies in depth and answer because the line preamp is a very important element of the audio chain, i think

Just one word now about valves
I think that valves are generally not accurate ... they usually add distortion
This is very evident in the distortion spectra of many tubed preamps and amps.
I find this unacceptable ... i cannot accept that i send in a let's say 1 kHz single tone and i get at the output a nice "comb" of harmonics.
I want just a 1 kHz output, maybe amplified in gain
I do not like harmonics generators also because this "invented" signals overlap the original signal.
I think that many of the "defects" of solid state sound depend on too little PS capacitance that translates in a feeling of dynamic compression
But let me read carefully all your very kind and valuable advice.
If i will be able to fix the line preamp issue i will be at a very good point in my search for decent sound.
Thanks again and kind regards, gino

 

[sesebe]

If you will use a very expensive coupling capacitor, for sure that you will hear an improuvement on sound but not neccesary because the capacitor it si very good but because it is expensive......

 

[AndrewT]

The posters are correctly differentiating between coupling capacitors that have negligible audio voltage across them and filtering capacitors that have significant voltage across them at some audio frequencies.

Use appropriate capacitors for these two quite different duties and you will not need to spend excessive amounts of money on "exotic" capacitors.

 

[ginetto61]

Hi now i am free ... like the Gladiator !
i was assuming, maybe wrongly, that the same supposed differences when used in xovers could be applied also at line level.
I am happy to know that is not the case.
Moreover i am a little scared about the complexity of dc coupled circuits that i think need servo circuits to mantained offset low.
So speaking of coupling caps what do you think about these ones here that are not expensive and also quite small as i have little space on the pcb ?

http://www.mundorf.com/english%201.1/kondensatoren.htm

An externally hosted image should be here but it was not working when we last tested it.

Thanks a lot again.
Regards, gino

 

[ginetto61]

This is the reality right here.
For a capacitor to introduce distortion, it has to have a voltage across it.
So if you select a coupling capacitor that has negligible impedance compared to the circuit it's driving, it will introduce virtually no distortion. So a 1 uF coupling capacitor for a 100 Kohm input impedance will have a negligible effect on distortion; its turnover frequency will be 1.59 Hz. This assures negligible effect at frequencies below 15.9 Hz.
An electrolytic that has no resonances below 200 KHz will be for all practical purposes transparent.

Hi and thanks and this is very welcome because one of those exotic caps costs more than my preamp
Actually the 4 red ones have been already replaced in my preamp ... but i like the white Mundorf better

In a filter, the capacitor has a voltage across it at a crucial frequency, and it will introduce distortion.
This is where careful selection of a capacitor is paramount, and this is where uber expensive capacitors can be justified.
Electrolytics are a terrible choice here (although commercial grade equipment uses them in this application), and metallized polyester film is a good choice if available in a useful value.
Cyril Batemann authored the definitive work on this topic

So in the end i could not be more wrong
Well very good to know ... i was fainting looking at the prices of the best ones
Speaking of voltage across i tried with the tester for curiosity but i have a very poor instruments ... non very precise
It looks low anyway. But i do not worry anymore .
Then if i will find a dc coupled preamp design maybe ... but i need a complete kit to have any chance to succeed ... i am afraid
Thanks a lot and kind regards, gino