AD1865 w/6DJ8 SRPP - mod suggestions

[lordearl]

Just finished listening to this (also see attached circuit) DAC for about a week and am looking for some suggestions to modify the board a little. I have some experience in changing capacitors and resistors, but don't really know about reading schematics.

Is anyone able to suggest what might be some worthwhile changes to the circuit?

[batteryman]

Originally Posted by lordearl

Just finished listening to this (also see attached circuit) DAC for about a week and am looking for some suggestions to modify the board a little. I have some experience in changing capacitors and resistors, but don't really know about reading schematics.

Is anyone able to suggest what might be some worthwhile changes to the circuit?

Replace the I/V transformers with ones from Sowter. The 9762.

SOWTER DAC I/V TRANSFORMERS

Sowter Transformer 9762 DAC INTERFACE* FOR AD1865N-J

The used ones in that kit will be the weak point.

I have a AudioNote Dac 1 kit which is also a NOS AD1865 design and I am fitting a pair of the 9762s to it in a few weeks.

Might also try them on an old Philips CD471 TDA1541 design as well.

I

[lordearl]

Wow - they look good, a designed especially for the AD1865, but are there any other cheaper options? Those transformers are more than the kit and chassis put together!

[batteryman]

Originally Posted by lordearl

Wow - they look good, a designed especially for the AD1865, but are there any other cheaper options? Those transformers are more than the kit and chassis put together!

They are expensive, but you already have cheap ones in the dac and wanted to upgrade the sound. This costs money if you want to make a worthwhile difference and I'm confident these will.

I

[lordearl]

Just took a snap of the transformer (attached) - it says primary 20kR and secondary 600R - how do I know if the Sowter is a suitable replacement?

[batteryman]

Originally Posted by lordearl

Just took a snap of the transformer (attached) - it says primary 20kR and secondary 600R - how do I know if the Sowter is a suitable replacement?

That's an easy one!
The Sowter one is designed for the AD1865 and there's no way the old valve era ones used in the kit were designed for use with any dac chips.

"For use with most current out Digital-to-Analogue Converters. Designed for and tested with AD1865N-J . Should also work well with AD1851, AD1852, AD1853, AD1861, AD1862 . Provides current to voltage conversion with approximately 22 dB gain. Presents a low impedance to the DAC output for best frequency response. Provides significant sound quality improvement when compared to other interface methods. Ensures full isolation of digital noise in ground and supply rails."

However I am assuming the ones in the kit are actually being used for I/V conversion and are connected directly to the dac chip.

The Sowter ones are intended for direct connection to the Iout pins of the 1865 (pins 4 and 21) so you'd need to follow the tracks from these pins to see where they go to. If not straight to the transformers, then disconnect from whatever they do go and connect instead to the transformer.
(a circuit diagram would be useful!)

[lordearl]

Thanks B'Man - from my limited knowledge I think there is a 220R resistor in between the DAC chip and the transformers (schematic is on the first post).

[batteryman]

Originally Posted by lordearl

Thanks B'Man - from my limited knowledge I think there is a 220R resistor in between the DAC chip and the transformers (schematic is on the first post).

Yes, I should have looked at the schematic first.

The Sowter ones will just replace the existing ones but you may need to experiment with changing R4 to the values suggested by Sowter.
The 220R on the dac output will be okay.

I

[stephensank]

A lot of old transformers like that can be as nice or nicer than the best you can get now, HOWEVER, those transformers are marked as TELEPHONE grade, 250-3500Hz +/-2dB. They must be quite a lot better than that as used in this app, but there's little doubt that a Sowter or Cinemag, etc. would be a lot better.

[lordearl]

Thanks again - I think the transformers are quite good (I have a solid state AD1865 dac and this DAC beats the pants off it!) but an upgrade should make a big difference again.

what does the "250-3500Hz +/-2dB" spec indicate?

re the resistors, I have altered R4 in the SRPP to 2k as there was far too much gain (resulting in distortion even!).