Cal123

AN DAC and Audio Innovations the First - impedance matching?

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I've been quite happy with the performance of my Audio Innovations 300, but an Audio Innovations the First power amp showed up for sale locally and I couldn't resist. This is the 4x ecc88 model, so the gain is unwieldy. I'm temporarily using it with my Chord Mojo, which is able to give a clean signal even at very low volumes. However, I much prefer the glare free treble of my Audio Note Dac 1, or even the Metrum Octave from my headphone rig. The previous owner used a Tisbury preamp for volume control and it seemed like a neat solution. However,AFAIK, the AN DAC likes to see an input impedance of at least 100 kOhm. The Tisbury spec sheet states a 10 kOhm for their preamp. So, my question is this: Is it possible to get, or make, a passive preamp with as much as 100 kOhm input impedance? Or might there be a different solution to this apparent incompatibility of the AN DAC and AI power amp?

edit: It appears my DAC has been updated to 1.2 specs, with Cca valves(ecc88/6922) and a resulting reduction in output impedance from 5 kOhm to "just" 1 kOhm. So the Tisbury 10 kOhm input impedance may suffice. However, I suppose 20 kOhm would be preferable. Would anyone know of a passive pre with upwards of 20 kOhm input impedance?

Edited by Cal123

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Super Wammer

Ah, the gain problems of the 1st and 2nd Audios. I resisted the advice from a person who knows (David Wright) for many years but eventually I tried what he suggested, a (edit) TVC, and have not looked back. Try a Music First preamp or something similar. Sit back. Enjoy. 

Edited by Fourlegs
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I am no DIY expert (you might need to ask one of the Pass DIY lads on here) but would a PassDiy B1 buffer pre work?

To the best of my knowledge it lets both the source and amp see impedances that they like.

https://www.passdiy.com/store

https://www.passdiy.com/pdf/B1 Buffer Preamp.pdf

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Fourlegs: Any chance you mean 'TVC', i.e. transformer volume control? And if not, is TVM something related? It's a compelling concept, that I wasn't aware of before. I particularly like the prospect of better SQ at low volume listening, which is what matters most to me. I could perhaps build a DIY TVC like the Django or the one provided by Glasshouse. However, would a TVC mate well with the high impedance output of the Audio Note DAC?

Omsoc: The Pass DIY would inevitably put a transistor into the signal path, wouldn't it? This may not bring the best synergy to my system. Would love to hear a Pass system one day, though.

Edited by Cal123

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As long as its properly designed a TVC should work with a source impedance of 1K. This assumes a reasonably high

primary inductance  ( typically 400H  ) so as not to cause LF roll off. Transformers ideally like really low source  impedances

to reduce distortion but I dont think 1K will cause any practical problem. Ive succesfully used AVC/TVCs with source impedances

of nearly 10K without adverse affect..

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The output impedance of AN's DACs is dominated by the output coupling capacitors. AN being AN, these tend to be exotic, expensive, and thus of a very low value. I seem to remember 1-2 uF, sometimes even less.

Such caps simply require a high input impedance in order not to roll off bass prematurely.

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Zeta4, that sounds encouraging! The STA-522A silk transformers, as used in the Django, have a claimed inductance of "up to +1,100 henry ". The Thai owner confirms that it will work properly with an output impedance up to 10 kOhm.

Now, STA-522A transformers have a max attenuation of -46 dB and the initial steps provide 40 dB, 34 dB, 32 dB, 30 dB. With the 100 mV sensitivity of the First amp, and running 92 dB speakers (AN-J), would those 6 dB initial increments allow for a comfortable adjustment at low level volume?

Werner: When you say that they output impedance of the DAC is dominated by the output coupling capacitors, wouldn't this effect already be factored into the 1 kOhm specification? I can see that later versions of AN DACs, I think starting at level 3, have output coupling transformers. I suspect these address any potential impedance issue, but I'm not sure whether retrofitting is a straight forward option.

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Super Wammer
23 hours ago, Cal123 said:

Fourlegs: Any chance you mean 'TVC', i.e. transformer volume control? And if not, is TVM something related? It's a compelling concept, that I wasn't aware of before. I particularly like the prospect of better SQ at low volume listening, which is what matters most to me. I could perhaps build a DIY TVC like the Django or the one provided by Glasshouse. However, would a TVC mate well with the high impedance output of the Audio Note DAC?

Sorry, yes I meant TVC. I use the Music First version with my Second Audios and it presents the impedence that the amps want to see and with therefore much improved sound quality compared to running the DAC direct into the amps.

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I've now managed to put together a TVC using a pair of Silk Audio STA-522A's, as used in the Django pre from diyhifisupply.The first thing I noticed after connecting the freshly soldered pre was how bass drums now have visceral impact. Also, my Audio Note Dac now appears to have much improved definition, where it sounded overly soft before. However, the minimum volume is much too high, particularly for vinyl playback. Initially, I thought I'd wired in the optional +6dB feature, but upon further inspection this does not seem to be the case (I've wired the two primary windings in series, not parallel). Thus, the attenuation at level one should be -48dB. Did anyone else find that this is not enough, and what might the remedy be? For instance, would it be possible to lower the gain of the AI First power amp without sacrificing sq?

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I'm not huge expert my any means but have been using second audios for a number of years now they are the early type ecc88. To get them to work with my pre amp (border patrol cu2) David Wright recommended using 6fq7 to lower the gain, I also use 6n23p as the line driver with pretty good results neither tubes are perticuly expensive. Good luck

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On 1/3/2017 at 20:03, Cal123 said:

For instance, would it be possible to lower the gain of the AI First power amp without sacrificing sq?

I use Rothwell 10dB attenuators with my AN DAC as they pump out a rather hefty 2.5V.

It is better to attenuate the hot source rather than the pre output as it will better align to other sources so you don't have to drastically change the volume going from digital to phono etc.

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I've now handed it over to an experienced technician, who will do an easily reversible modification that should solve the problem. I plan to report back.

UPDATE: His plan implied some capacitors, which, as it turns out, will not fit in the amp. So he suggested simply installing a set of resistors. As I use a transformer volume control, an input impedance drop shouldn't cause problems. 

Edited by Cal123

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On 1/3/2017 at 22:55, Youngman said:

I'm not huge expert my any means but have been using second audios for a number of years now they are the early type ecc88. To get them to work with my pre amp (border patrol cu2) David Wright recommended using 6fq7 to lower the gain, I also use 6n23p as the line driver with pretty good results neither tubes are perticuly expensive. Good luck

 

From what I was able to find on the ecc88 vs 6fq7, the latter would probably reduce gain with about 3 dB. I think I would need at least a 6 dB reduction. Does the 6n23p reduce gain further?

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Hi no the 6n23p has more gain than an ecc88 very similar to the e88cc. Since my last post i have purchased an audio note dac and have faced the same problem i have fitted 47k resistors to one of the inputs, I used tacman carbon film type resistors and it sounds fantastic! 

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If you replace all four ECC88 with 6FQ7/6CG7 that will give you a 6dB reduction.

If you get US brand 6FQ7/6CG7 the amp will also sound sweeter and less glassy than with the ECC88's.

The Ei 6FQ7/6CG7 will also work but it doesn't sound that good.

This is the cheapest, simplest option and I would do it regardless of gain issues because IMO it sounds better.

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