Borberly Op amp 4 Tube Mosfet hybrid

[Mayday]

Hi,
I'm building and collecting parts for the Borberly Op amp 4.
I have two good Philips ECC86 and all the transistors, the rest of the parts are on their way.

This only my second attempt at building with tubes, except for fixing a small tube amp.

I've opted to go for Allen Bradley resistors(but also have Vishay and Roederstein at home), Silver Mica for 22pF, Wima's for 100pF. Don't know if polystyrene would be better caps in this application? I will populate R15 & C5(100pF I read somewhere, can't remember where atm), is there any drawbacks to this?

This circuit is to be used as output on a DAC directly after passive I/V and followed by a matched pair of NOS 2uF russian PIO's.

I've got a 2x24VAC transformer rated at 30VA(625mA if my math is somewhat correct), I hope this will be enough for the two ECC86 and the two pairs of 2SK216/2SJ79 (not matched btw).
Filament supply will be handled(if possible) by a 5VA 0-6VAC transformer (220V primary, we have 230V mains).
I'll have to measure it when it arrives to see what the secondaries actually deliver. If too far off 6.3VAC, I'll use a bridge rectifier and LM317 set to 6.3Vdc.
If wiring the filaments in parallel to the PS, 6.3V is ok I suppose but if wiring them in series I'd need 12.6V? As stated I'm fairly new to this.

For those of you that do know this stuff, is there any issues with DC offset/drift with this design? I assume the 100R trimpots are for trimming DC as no two halves of any tube is 100% identical.
If there is issues with DC offset, how have those of you that have built this circuit handled it?
The easiest, but I doubt the best, way would be to swap the 200R (or 165R?) for trimpots I assume?

For the current source, I have two J511's on their way, they should to the job?

What are your opinions of the sound quality of this design?
I'm hoping it'll atleast beat opamps and maybe the ECC86/2SK170 SRPP hybrid I have in another DAC.

 

[indra1]

Do you need the ~19dB gain? If can live with less gain, an ECC86 into Lundahl LL1680 can be configured to give ~2dB (150 ohm output impedance) or ~8dB (600 ohm output impedance) of gain. I believe it is hard to get a better sound from a single triode into an LOT. Since you are in Sweden, you don't have to pay import duties and exorbitant shipping cost.

 

[Mayday]

Do you need the ~19dB gain? If can live with less gain, an ECC86 into Lundahl LL1680 can be configured to give ~2dB (150 ohm output impedance) or ~8dB (600 ohm output impedance) of gain. I believe it is hard to get a better sound from a single triode into an LOT. Since you are in Sweden, you don't have to pay import duties and exorbitant shipping cost.

I wish I could afford Lundahl transformers, sadly that will have to be something for the future.

 

[indra1]

How so true. Some people even say that an amp is a project but a good transformer is an investment.

 

[erikb]

Parallel heaters will keep the voltage right (6V should be fine), but the current from the transformer will double. 5VA might be on the small side (for a stereo amp), if the transformer reaches it's limit it will get hotter and the voltage will drop.
If you rectify the 6V and use a lm317 regulator, the 5VA wil definitly be to small, because the lm317 will eat current too.
Also read The Valve Wizard, to prevent hum problems.

 

[Mayday]

5VA 0-6VAC, it's on the small side?
Writing from my phone, so I don't have the ECC86 datasheet in front of me.

 

[Mayday]

According to the datasheet, heater current for ecc86 is 330mA.
Two in parallel should draw 660mA.
833.33mA might not be enough headroom?

 

[Mayday]

BOM advice

Parts I have or that are ordered

10K resistors: IRC RND60C
499R resistors: IRC RND60C
200R resistors: AB 1/2W
100R resistors: AB 1/8W (enough?)
1K1 resistors: AB 1/2W
100R trimpots: Bourns
165R resistors: Vishay 1/4W
47.5R resistors: Vishay 1/4W
22pF: polystyrene 5% 160V
100pF: polystyrene 5% 50V TSC
220uF: Nichicon PW
4.7mA current source: J511
Tubes: Philips ECC86

I have Vishay, Roederstein and RLR5C as options to the resistors above.
Wima FKP2 as option to 100pF, Silver Mica and "regular" ceramic as options for 22pF.

This circuit will be used as output after single/parallel TDA1541A dac with passive I/V.
Passive I/V as below
Single dac: 47R Holco & 47nF CDE axial MKP
Parallel dac: 22R Riken RMG or 20-22R Holco & 100nF russian NOS PIO's.

Between the "opamp" and RCA connectors I'll put a pair of matched 2.2uF NOS russian PIO's.
Yes, I do like the russian NOS PIO's

Wiring is done using mil.spec. RG178 throughout the build. One end of the screens is connected to GND in all wiring.

Any opinions or advice regarding the parts I've picked?

 

[mkusan]

5VA 0-6VAC, it's on the small side?
Writing from my phone, so I don't have the ECC86 datasheet in front of me.

For your heaters I would try to find transformer with higher secondaries, like 8V, if you plan to use DC that is. Especially if you plan to regulate it with the likes of LM317, LT1085... your voltage in-out difference could be on the low side to regulate properly. If you already bought the transformer, use Schottky diodes for a bridge, because of lower voltage drop.


Kind regards

Marko

 

[Mayday]

For your heaters I would try to find transformer with higher secondaries, like 8V, if you plan to use DC that is. Especially if you plan to regulate it with the likes of LM317, LT1085... your voltage in-out difference could be on the low side to regulate properly. If you already bought the transformer, use Schottky diodes for a bridge, because of lower voltage drop.


Kind regards

Marko

Hi,
I'll use AC if the output voltage is ok when I test the transformer.
Yes, I've already recieved the transformer.

Regards,
Jonas

 

[Mayday]

I just thought of a thing,
The circuit is differential input, the output from the Red Baron board (TDA1541A) are L G R.

I guess I should use the non-inverting inputs for the audio signal?
How do I connect the inverting input?

 

[Mayday]

I am thinking of using a regular LM317/337 voltage regulator for this circuit, the +/- 24Vdc that is. Filaments will be AC as long as output voltage of the transformer is ok and there is no noise issues.

I know there are better alternatives out there, but honestly the enclosure for the DAC this will, hopefully, work as output in is getting very cramped.

The LM317/337 is "good enough" for this circuit?

 

[Kay Pirinha]

This circuit is to be used as output on a DAC directly after passive I/V and followed by a matched pair of NOS 2uF russian PIO's.

Sorry, but I don't see the holy grail in these paper-in-oil caps. *Imho* any modern plastic foil cap will do a better capacitor job than that old-fashioned boys.

Best regards!

 

[Mayday]

Sorry, but I don't see the holy grail in these paper-in-oil caps. *Imho* any modern plastic foil cap will do a better capacitor job than that old-fashioned boys.

Best regards!

I just like how they sound

 

[Deka]

@MayDay

Have you finished this amplifier finally?

 

[Mayday]

No, I got fake transistors

 

[Deka]

I'm still wondering, how this amplifiers might sound. Do they have tube character? Does it worth the effort to build them?

 

[Steve Morley]

I built these years ago from kits I ordered directly from Borbley. I memory serves they were OK but not earth shattering. The ECC86 is OK but a bit too laid-back for my tastes. Low voltage tubes like that often are, IMHO. It can be configured to use ECC88s with a bit more voltage. I have two un-built kits, boards and all the parts, tubes too if anyone would like to buy them.

Steve

 

[dady]

Hello, this design aims to bring out the best in both worlds, tubes and mosfets. Erno Borberly made reliable designs the entire time. The idea is fine, but if the transistors you are looking for are scarce, also, if you get them, you will have to match them in any way. The other problem I see is that you have a triode amp, but the output is Common Source and the curve you get is not triode. Maybe you have to get a hybrid with Source Follower and also with more obtainable as possible. Also with the output follower you will have a better adaptation with the speakers due to a greater damping. You will have a transparent output, without voltage gain but with the linearity of the previous stage which is a triode. Another problem would be that the ECC86 does not have enough gain to get all the voltage amplification either.There are many circuits with normal Mosfets transistors both vertical and lateral ones. About the transformer you have, if you rectify the 5volt you will get 7.05 volt that you can down with a resistor, a 317 needs more voltage for work. Use at 6.3volt 1amp. For the main power supply you can use the 24volt for the valves but you need some more power for the output. At least an 20 + 20volt 100VA. The rest of the hi end components that you comment on will not improve anything on the circuit.