TDA1541A Kompakt II with tube output stage

[Gieseler Audio]

I have been kicking this idea around in my head for ages & now I think it is time to make it happen.

I have a ton of experience in this area so design/application should be no problem.

 

What really got the ball rolling was when my attention was drawn to the ECC86/6GM8 tube.

This is a 1960's era tube designed for low voltage car radio applications.

It is very similar to a ECC88/6DJ8 (twin triode) but designed for very low voltage operation.

The ECC86 is intended to operate with anode voltages of around 6/12/24 volts.

I will probably go for 24v to maximise output & that will be easily obtainable from the current Kompakt II external 

transformer with a simple voltage doubler circuit.

 

The plan is to use the current Kompakt II design/case but with tubes replacing the current opamp output stages.

Imagine the current Kompakt II but with two tubes poking through the lid -  cool!

I have found some 22mm rubber grommets so I just need to drill two holes in the lid & the fit the grommets for a tidy exterior look.

The tubes will be user replaceable without removing the lid.

 

Tube design will be the classic common cathode triode gain stage followed by a cathode follower output stage.

This will keep the output impedance nice & low so compatibility will be good with most amplifiers.

Now ECC86's are very old but there appears to be plenty of new old stock available and at reasonable prices.

The other option is if I raise the voltage slightly to around 30/40v I could then use a ECC88/6DJ8 which is in plentiful supply.

I will try both options & see what sounds best.

 

Cost;

Well it will definitely be a jump on the current Kompakt II but hopefully not much & should be under $3K

 

I am designing a PCB right now so I should have some prototypes up and running in a few weeks.

Stay tuned!

 

[bjs]

@xlr8or Would love to hear your thoughts on the tubes

[xlr8or]

1 hour ago, Gieseler Audio said:

I have been kicking this idea around in my head for ages & now I think it is time to make it happen.

I have a ton of experience in this area so design/application should be no problem.

 

What really got the ball rolling was when my attention was drawn to the ECC86/6GM8 tube.

This is a 1960's era tube designed for low voltage car radio applications.

It is very similar to a ECC88/6DJ8 (twin triode) but designed for very low voltage operation.

The ECC86 is intended to operate with anode voltages of around 6/12/24 volts.

I will probably go for 24v to maximise output & that will be easily obtainable from the current Kompakt II external 

transformer with a simple voltage doubler circuit.

 

The plan is to use the current Kompakt II design/case but with tubes replacing the current opamp output stages.

Imagine the current Kompakt II but with two tubes poking through the lid -  cool!

I have found some 22mm rubber grommets so I just need to drill two holes in the lid & the fit the grommets for a tidy exterior look.

The tubes will be user replaceable without removing the lid.

 

Tube design will be the classic common cathode triode gain stage followed by a cathode follower output stage.

This will keep the output impedance nice & low so compatibility will be good with most amplifiers.

Now ECC86's are very old but there appears to be plenty of new old stock available and at reasonable prices.

The other option is if I raise the voltage slightly to around 30/40v I could then use a ECC88/6DJ8 which is in plentiful supply.

I will try both options & see what sounds best.

 

Cost;

Well it will definitely be a jump on the current Kompakt II but hopefully not much & should be under $3K

 

I am designing a PCB right now so I should have some prototypes up and running in a few weeks.

Stay tuned!

 

 

More than happy to beta test different 9-pin tube types.

 

You may also want to consider using the ECC85/6AQ8 which is still plentiful, reasonably priced, and available both in vintage NOS/UOS and new production. It's a wonderful sounding tube without attracting a high cost.

[Gieseler Audio]

Thanks  - will check those out

[Gieseler Audio]

Well finally after lots of designs, tweaking, testing I finally have a nice working design.

The big challenge was to get a dual twin triode tube line stage all working nicely from just

a single 12v external transformer.

The same 12v transformer will also have to power the TDA1541A DAC.

The usual criteria for my designs is to power them from Australian approved, low-voltage external transformers & this is achieved with this design.

Tubes  -  ended up running with 6922/6DJ8/ECC88 which is one of my favourites & in plentiful supply in new & old stock.

The tube stage measures really well so the next step is to actually try it with the TDA1541A DAC as a source & compare the sound to the original  TDA Kompakt II.

Will certainly be interesting.

Stay tuned!

 

 

 

[RockRolley]

Congratulations and thanks for sharing. FOMO

[Gieseler Audio]

Update!

 

Well finally I have conducted some actual listening tests with the TDA Kompakt II using the tube line stage rather than the usual solid state output stage.

First impression was a little underwhelming, no point in going to all the trouble of using tubes unless there was a noticeable improvement over the regular version.

I nearly gave up at this stage but stirred on by the challenge I spent several days tweaking & fine tuning the tube stage.

 

There are a lot of configurations, Common Cathode/Cathode Follower, Single Common Cathode, constant current sources added in & SRPP.

I finally settled on Common Cathode with both triode sections in parallel.

This sounded by far the best.

Thorsten Loesch (AMR design fame) came to the same conclusion so I was happy with my decision.

 

Output impedance is up a bit compared to Cathode Follower but as long as your amp/preamp has an input impedance of 10K or greater there should be no issues.

Finally it was starting to sound really nice so I started comparing different ECC88/6DJ8/6922 tubes.

Then I popped in a pair of Genalex Gold lion E88CC's 

OMG!  -  This was the wow factor I was looking for.

They are $200+ for a matched pair but well worth it as the sound with them is amazing.

 

Next challenge was to redesign the Kompakt II pcb to accommodate the tube stage & TDA DAC

A rather daunting task.

 

It was certainly a challenge but after several days I have finally completed it & ordered the PCB's.

They should be here early next week so I will post a photo of the final design then.

Stay Tuned!

 

 

Edited March 31, 2024 by Gieseler Audio

[MattyW]

Mmmmmm. I'm curious, what price point will you be looking at with this Clay with a matched pair of Genalex Gold lions installed? Seems silly for me to consider for a secondary system but here I am.... 

[Gieseler Audio]

Hi Matty

I have gone through the extra build time, parts costs etc and $3K is looking pretty accurate.

That will include  Genalex Gold Lions installed as standard.

It is a much more complex build with the addition of tubes, case drilling ect & increased parts cost so that bumps the price up quite a bit compared to the regular solid state version.

I should have a production version running early next week so I will do a sound comparison with the regular version and see if the extra price can be justified.

Unfortunately I am also heading off to Japan for two weeks late next week so that's going to delay things a bit.

 

 

[MattyW]

I believe even the regular version has been compared favourably with the Abbas Audio DAC's which is already a rather high benchmark so the tube output version of your DAC may be very special indeed. It will be interesting to read about your development of this.

 

I hope you've a nice trip to Japan. 
 

Edited April 3, 2024 by MattyW

[lenticularis]

Enjoy your Japan trip Clay!

[Gieseler Audio]

Thanks guys

Really looking forward to the Japan trip 

This time next week I'll be there

[Gieseler Audio]

OK just back from Japan & now have a production board assembled.

It still requires a bit of fine tuning to optimise the sound quality 

but it is definitely sounding very good.

 

 

 

 

[RockRolley]

If anyone near the Mornington Peninsula snaps one up, I'd love to catch up and do some A/B listening with my solid state version

 

[Gieseler Audio]

Fine tuning complete & it is sounding very nice!

However I am having a ton of fun playing with tubes again so I 

have decided to try one more variant before I commit to the final design.

I am a great admirer of Thorsten Loesch  - part of the Abbingdon Music Research design team.

Anyway Thorsten is a big fan of choke plate loads for tube line stages and used that format for a TDA1541A DAC many years which received great feedback so I thought I have a go at that.

It was a little tricky sourcing a suitable choke that would suit pcb mounting but I finally found one.

Initial testing went very well so I have actually ordered some PCB's which will use the chokes.

It is a single tube design so one triode section for each output stage.

Often a single triode stage can sound the best but it's harder to get a low output impedance.

I have also incorporated 6v/12v tube heater selection so there will be lots of tube options.

Once I get this version built I will do a comparison with the previous one & report back.

Stay tuned!

 

 

Edited May 5, 2024 by Gieseler Audio
Grammer

[Gieseler Audio]

Well in the listening tests the twin tube version was a clear winner.

Also as it uses parallel triodes it had a lower output impedance which is a plus.

Obviously there is no balanced outputs  as this would involve two more tubes & a lot more complexity and cost.

 

I ended up doing a few more tweaks as well to improve performance.

1.   Nice IXYS CCS's (constant current sources)  for the plate load replacing the standard resistor.

As the HT voltage is fairly low the CCS enables about another ten volts on the anode of the tube which improves performance.

2.  Lower value I/V resistors for the TDA1541A  -  a direct result of the CCS which increases gain.

3.  Additional regulation for the TDA chip.

 

Tubes will be E88CC Gold Lion but  there is also an option to set the heater voltage to 7v to suit PCC88's.

I have had a good critical listen & I'm very happy with the final result.

There is a tiny bit of noise with your ear right up close to the speakers which is normal for tube gear.

Photos below  -   note still waiting on hole liners where the tubes protrude through lid.

This is a long fiddley build and complex set up so I don't think I will list it as production model but it will be available as a special order.

Cost  $3000.00 

 

 

 

 

 

 

 

 

[MattyW]

Hot damn, I’m so tempted by this Clay…. I don’t need it per se but it has me near drooling  

 

If I do order I’ll probably get it sent to my mate @muon* then once he’s had a listen on to some mates in VIC before I get it back as they’d love a listen for sure. This is a very special bit of kit. I may end up pairing with my Talisman T33H headphone amp and Verum II headphones (once I get them) in my office computer setup. Toggle that and the amp on when needed.

Edited May 29, 2024 by MattyW

[RockRolley]

1 hour ago, Gieseler Audio said:

Well in the listening tests the twin tube version was a clear winner.

Also as it uses parallel triodes it had a lower output impedance which is a plus.

Obviously there is no balanced outputs  as this would involve two more tubes & a lot more complexity and cost.

 

I ended up doing a few more tweaks as well to improve performance.

1.   Nice IXYS CCS's (constant current sources)  for the plate load replacing the standard resistor.

As the HT voltage is fairly low the CCS enables about another ten volts on the anode of the tube which improves performance.

2.  Lower value I/V resistors for the TDA1541A  -  a direct result of the CCS which increases gain.

3.  Additional regulation for the TDA chip.

 

Tubes will be E88CC Gold Lion but  there is also an option to set the heater voltage to 7v to suit PCC88's.

I have had a good critical listen & I'm very happy with the final result.

There is a tiny bit of noise with your ear right up close to the speakers which is normal for tube gear.

Photos below  -   note still waiting on hole liners where the tubes protrude through lid.

This is a long fiddley build and complex set up so I don't think I will list it as production model but it will be available as a special order.

Cost  $3000.00 

 

 

 

 

 

 

 

 

 

Edited May 29, 2024 by RockRolley

[Gieseler Audio]

Update  -  there were some noise & hum issues that I could never totally solve

so I have discontinued this model.