Using the AD844 as an I/V

Sparkos Discrete Op Amp Review

I had a chance to hear my version of the PCM1704 DAC in my cousins system yesterday. He has the Alon `4 loudspeakers. So has a lower bass frequency response then I can get with my ribbon planar MG2.5R based Magnepan system. Listening impressions: Bass. Has a clean powerful low end. Even more so then the BUF03. A sense of limitless power. Kick drums have weight and a just correct quality. Midrange is clear and vocals are absolutely defined in space a "you are there" quality. Treble is clean and absolutely real sounding. Good recordings leave one to just close your eyes and listen. The SS3601 clearly out performs the OPA627 and OPA1641. The transparency and holographic presentation betters these op amps. Extra detail is also heard. I'd recommend this discrete op amp in critical applications. It's cost is more then the venerable OPA627. However not by that much all things considered. PCM1704 DAC... It is my DIY reference which all others must be compared with. I don't plan to modify or change this going forward. I am fortunate that I have another pair of PCM1704's that are ungraded. So I can experiment with those in the future. The best testament I can give... I am more interested in listening to music then rolling op amps in the reference DAC. :cool:
 
I tend to prefer the transformer approach over the active filter one. Adding additional active devices in my experience does more harm then a quality transformer does. My experience might not be typical. :)

Its been my experience too. I definitely want to minimise active devices in the audio chain.

I had an Audio Note Dac1 a few years ago - AD1865 in nos mode, no filtering, tube output stage. I couldn't live with the aliasing and birdies until I tried my Sowter transformers.
What an improvement with no loss of treble or any other side effect. Not perfect of course, but the benefits far outweighed the cost.

I had the same results with a pair of parallel 1541s and hope for an even better result when I have them differentially connected (due to cancellation of common mode noise), initially with just passive I/V and then with single or stacked 844s.

I will even try the transformers between the + & - Tz resistors.

The transformers have 5:1 step up (+20db) so there is no need for Tz resistors of more than 100R (same value I used when I had passive I/V).
 
Sallen Key

I was more thinking to use the existing stages to add filtering to, not add an additional one.

Cheers George

In that case it starts to look like a non-inverting Sallen-Key filter. A better option is a GIC filter assuming the DDNF can drive it and the load is high enough going into the Buffer. The GIC gyrator configuration places the filter function in a shunt with the signal. The quality of the op amp is less an issue. However you likely would need 2 parallel sections to equal what the Lundahl is actually doing. I suppose that is a good area to explore. ;)
 
Transformers and parallel DAC's

Its been my experience too. I definitely want to minimise active devices in the audio chain.

I had an Audio Note Dac1 a few years ago - AD1865 in nos mode, no filtering, tube output stage. I couldn't live with the aliasing and birdies until I tried my Sowter transformers.
What an improvement with no loss of treble or any other side effect. Not perfect of course, but the benefits far outweighed the cost.

I had the same results with a pair of parallel 1541s and hope for an even better result when I have them differentially connected (due to cancellation of common mode noise), initially with just passive I/V and then with single or stacked 844s.

I will even try the transformers between the + & - Tz resistors.

The transformers have 5:1 step up (+20db) so there is no need for Tz resistors of more than 100R (same value I used when I had passive I/V).

Hi Ian, Sounds like we are on the same page. Although your well ahead of me on the I/V conversion using transformers as a step up device. That actually should work very well. I have a Cinemag microphone transformer that was sent to me by mistake. I'd need a second one to make an attempt. I'll have to look at that and see what the turn ratios might be. :cool:
 
Has any one tried these transformers on eBay claimed to be made for DAC outputs?



s-l225.jpg


  • For Passive I/V of DAC
  • Low Copper Resistance
  • Low Distortion
  • High Level Capability
  • Nickel Permalloy Core
  • Mu-Metal Shield
1. 20 Ω : 600 Ω (CT 150Ω) 1:5.48(2.74)
2. Distortion (THD) : <0.001% @1KHz
3. Frequency Response : 5 ~ 100KHz(-3db)
4. Power Level : MAX +24dBu @20Hz
5. Transformer Size : 30 x 24 x 18(W x D x H : mm)
 
Torchwood421,
thank you for tell us your Impression with Sparkos Lab discrete op amps - It Is on my list to try
I guess you are using 3 x AD844 for I/V and then Sparkos as a buffer ?
but why there Is a transformer In your set up


I think he mentioned in the previous post. The trannies is for low-pass filter or maybe a passive gain stage. I also ordered the Sparkos. I will compare it with JG buffer and other discrete op amps. First thing to do, I need to increase my supply voltage to +-15V.
 
Torchwood421,
thank you for tell us your Impression with Sparkos Lab discrete op amps - It Is on my list to try
I guess you are using 3 x AD844 for I/V and then Sparkos as a buffer ?
but why there Is a transformer In your set up
IIRC, he is now using Pedja's DDNF I/V.
The transformers I believe are filters, filtering out HF noise. I could remember incorrectly though, the man does have 5 DIY DAC's ;)
 
Another bonus for transformers on the 844?

I had an inspiration yesterday. I've been trying a JLH class amp (high power version) but its not up to driving my Martin Logan Aeons other than at low to medium volume.
I don't want to start again with a Passlabs F5 or such, then I had the inspiration when I remembered my I/V transformers have dual bifilar wound secondaries.

I can build a identical JLH second amp and use it in bridge mode simply by reversing the polarity of one of the secondaries to the other amp. I will need a dual gang pot per channel with very good channel matching but it also means I can reduce the standing current through the amps to less than frying temperature. Its win/win for the humble transformer!
 
DDNF

Torchwood421,
thank you for tell us your Impression with Sparkos Lab discrete op amps - It Is on my list to try
I guess you are using 3 x AD844 for I/V and then Sparkos as a buffer ?
but why there Is a transformer In your set up

Hi samoloko, No. I am using Pedeja Rogic's DDNF I/V. It is a discrete current mirror. He designed this so you could have more current in the mirrors then a AD844 can provide. Coupled with a good buffer it is capable of out performing a triple stack. So I am using the Sparkos to buffer the I/V as a unity gain device. The transformer acts as a high frequency rubbish filter. Starts to roll off at 21 Khz. It works quite well for that. Does less harm to the signal then a Sallen-key filter does. In fact it is hard to tell any difference between Lundahls , Cinemags and Edcors as I apply it. :)
 
Doesn't Pedeja's DDNF have a point just like the 844's TZ point, where he takes for memory a 3kohm to ground to set the I/V's gain? You can utilise this for the LP just like the TZ resistor on the 844 with a cap across it, and ditch the transformer. Which I find OK to listen to if they have no gain, but as soon as they step up they have problems, I found this with ones they use as TVC's. OK at unity gain still coloured though, but rang like bells if gain was used.

Cheers George
 
Doesn't Pedeja's DDNF have a point just like the 844's TZ point, where he takes for memory a 3kohm to ground to set the I/V's gain? You can utilise this for the LP just like the TZ resistor on the 844 with a cap across it, and ditch the transformer. Which I find OK to listen to if they have no gain, but as soon as they step up they have problems, I found this with ones they use as TVC's. OK at unity gain still coloured though, but rang like bells if gain was used.

Cheers George

That's a rather sweeping generalisation and not my experience with the Sowter 5:1 step up ones I use. Maybe the 5H inductance has something to do with it.
If there was any ringing, its easily controlled by adjusting the secondary load to damp it out or using a Zobel network.
 
That's a rather sweeping generalisation and not my experience with the Sowter 5:1 step up ones I use. Maybe the 5H inductance has something to do with it.
If there was any ringing, its easily controlled by adjusting the secondary load to damp it out or using a Zobel network.

Just going by the way TVC's looked on the scope, with and without step up.

Cheers George
 
DDNF and transformer ringing

Doesn't Pedeja's DDNF have a point just like the 844's TZ point, where he takes for memory a 3kohm to ground to set the I/V's gain? You can utilise this for the LP just like the TZ resistor on the 844 with a cap across it, and ditch the transformer. Which I find OK to listen to if they have no gain, but as soon as they step up they have problems, I found this with ones they use as TVC's. OK at unity gain still coloured though, but rang like bells if gain was used.

Cheers George

Hi George, There is a point on the DDNF that is an I/V resistor and there is a capacitor. I do utilize a single pole or first order filter there. It isn't enough in my system. So the passive filter is needed. I now use the transformer as 1:1. Ringing... I did some experiments. If you do not properly load the transformer then ringing is the usual outcome. Properly loaded it presents a stable load. Gain may change that. Sonic signature... I find it more transparent then the usual active filter. The Sallen-key... Yuck!
 
How does a 1:1 transformer filter out the higher frequencies? Is it just an incidental bandwidth limitation?

Does it do it without phase shift?


All transformers come with bandwidth limit, and that's the characteristic which filter out HF content. I have no experience with transformer, but I was told the LF is the key for audio. Usually transformer will have distortion on LF. The primary inductance is important here. BTW, I think all forms of filter will have phase shift. It's inevitable.