Adding line pre-amplifier for my 300B Tubes

[Hari Iyer]

I had this project in Mind now for almost 9 months and now i think it will find its light. Nothing fancy but using a simple op-amp and a class A stage for the output increase the voltage gain of the source. Will keep the quiescent current at around 80mA to 120mA range depending upon how much heat the output stage can take. Should take 2 to 3 weeks to complete and adjust.

 

[Hari Iyer]

Got he artwork ready from the vendor. Will start building this weekend.

 

[Hari Iyer]

Completed the build and design goals have been met.

 

[viren bakhshi]

Hello Hari,

Now that you have a good tube power amp, it would be a shame to compromise the sound with an opamp based preamp.

Where tubes really shine is in handling low level signals. So, try putting together a simple tube line preamp - wire point-to-point, no PCB required. Then, sit back and compare the two. You will be surprised!

Here's a good start: DIY 12AU7 Tube preamp

Regards,
Viren

 

[Hari Iyer]

In my CD-player there is an op-amp used as low-pass filter after the DAC, also in my phono pre-amp (RIAA) there is an op-amp used there too. Was wondering how much of a difference will be if i used a tube pre-amp after these already used op-amps.

I have used an audiophile grade op-amp LM4562 (Texas Instrument) with vanishing low distortion of 0.00003% and have used a class-A output stage with a quiescent current of 60mA which is quite high for a line amplifier. I have used a gain of around 3.3 (amounts to 11dB) for the line amplifier. Given this i can say the line amplifier is almost a straight wire with gain not adding anything to the sound signature but just plain amplification. I have used a dual power supply with LM317 & LM337 chips with current and voltage regulation with high value capacitors with further filters in the line amplifier. All resistors are 1% MOR type and all capacitors in the signal path are polyester type. Will be replacing the BD139 and BD140 transistors with a better one MJE243 & MJE253 say after a month or two to further improve quality after the circuit passes the stability test.

 

[viren bakhshi]

Hari,

Nothing much you can do with the output circuitry of the source players.

But the preamp is in your hands. How do you think the opamp gets those astonishingly low distortion numbers - it has gobs of gain, and gobs of feedback. Try a single gain stage circuit, without feedback, which tubes allow, and hear the transparency in the sound.

Regards,
Viren

 

[Hari Iyer]

Until i borrow a tube line amp for a day or two for doing a apples to apples comparison I cannot comment further. I can for sure say that the sound offered by the 300B tubes is a shade better than the Sony and Denon amps because of class A stage.

 

[Fantastic]

You should test your new preamp in and out of circuit to determine if it's colouring the sound. Don't bother about all those 'vanishing distortion' figures. It doesn't tell you much about what you 'actually' hear !

Setting up your system to plug your preamp in an out of circuit must be done carefully. The load on your source should not change. Might require some parts value fiddling and switching tricks. If you can't maintain all load values the test will not be meaningful. Not to mention that the preamp must be accurately set for unity gain.

But seeing your circuit and having tried similar things in the past I'd agree with Viren that you might be better off with a simple tube stage. You'd need to do this properly and not just 'throw' any single stage tube at the system. If you are not careful you can get ' not so great sound' even with tubes !

I notice you are using large polyester caps at the input and output ! Maybe you should try out a good set of Polypropylene caps. You should get a significant improvement in sound. With the tube preamp you can have just one cap in the signal path !

 

[Hari Iyer]

@ Fantastic
I may not be able to find out myself if the pre-amp is coloring the sound. Since it sounds louder (+11dB gain) i may find it more better than the previous setup. In my earlier setup i took the pre-amp out from the Sony amplifier and fed it directly to the 300Bs. But the gain offered by the Sony was not enough and hence i had to jack up the volume to almost 3 o'clock position to get reasonable SPLs. This caused it to clip at high levels in some recordings. Now i can say for sure that this problem is not there as now i can set the volume control in Sony at just 9 o'clock position to get decent SPL. Hence now both the pre-amp the 300B tubes have enough headroom before the on-set of clipping. This itself must have reduced the coloration to a very large extend.

For the dc blocking caps, i have kept a 1microfard polyester capacitor for the time being. Will replace them if at all required. I am not getting any major dc voltage at the output and hence can also avoid them totally. My dc voltage at o/p is always less than 1mVolts. After i replace the transistors with MJE series will by-pass the capacitors too. Also will add a 1Mohm resistor at the input impedance and a 100Kohm resistor at the output impedance. In the distant future am planning to replace the op-amps with a JFET input transistors.

Cheers,

 

[Fantastic]

There is too much to say after your last post !
Don't have time to type it all out !

Maybe the best thing is to remove the input and output caps and enjoy the sound for several weeks at least BEFORE thinking of doing something else ! You always need enough time between serious changes. If this turns out to be very good as it is, why change it again ?
It doesn't have to be technically perfect. If YOU like it very much, that's all that matters ! Sit back and enjoy your music !
Cheers.

 

[Hari Iyer]

Building a rosewood finish wooden case for the preamp which should be ready by next weekend.

 

[omishra]

This thread is useless unless DIY community sees pictures and circuit. Hari, please post something which we can enjoy. Performance goals achieved - does not excite much.

 

[Hari Iyer]

This thread is useless unless DIY community sees pictures and circuit. Hari, please post something which we can enjoy. Performance goals achieved - does not excite much.

@ Omishra.

i think the very first post shows the circuit diagram designed by me with the component values. Also currently i am using a stock case from scrap dealer which i am planning to replace with a rosewood case which will be ready in next 15 days. The images will be posted after the build.

Performance wise its the best pre-amplifier which i have heard till date. But i can boast tons about the same but FMs will say that i am boasting. Hence it will be better if they themself find out about that. The circuit is class A output stage with a quiescent current of around 50mA. The op-amp used is an audiophile grade one with astonishingly low distortion. If you happen to be around Thane location, do drop for a listen.

 

[aks07]

... But the gain offered by the Sony was not enough and hence i had to jack up the volume to almost 3 o'clock position to get reasonable SPLs. T

What happens now when you crank up the vol pot to 3 o clock position?

Frankly I don't see the need for a linestage. What is the input sensitivity of this amp? My little experience with these things says that the lack of SPL is not due to input but at the output, e.i, the speakers, listening environment etc. This assumes you are using a CD player as source.

Secondly, I don't know what kind of driver circuit and OPT this amp is using. Its easy to clip 300B if the driver topology is not robust. Most budget Chinese 300B schematic I have seen take a lean approach on this. Left to me I would gut out this amp and rebuild with a different driver tube/topology.

Just my humble thoughts. I really wouldn't dare to cross swords with a celebrated designer like you.

 

[omishra]

Hari, thanks for pointing it out. We will wait to see end results.
BTW, my passive pre sometime need cranking up to 3'o position, where normal listening is at 9'o. That does not means its bad. You need the gain in pre as much as max volume could get your power amp to clipping. More gain and choosing low level at volume control means you are throwing source signal out and adding amplification with added noise of preamp itself.
Thats my thought!

 

[Hari Iyer]

I am now directly connecting the CD output to the pre-amp which has a gain of 9dB after all the tweaks. It was quite easy to adjust the gain with the op-amp as i need to adjust only the feedback resistor values.

For most of my well recorded CDs i keep the volume at 9 o'clock position which comprise around 70% of the listen. For the rest 20% i will crank the volume at 12 o'clock position. On some cases where the recording level is low i have played at full volume with no issues.

Also the headroom offered by this pre-amp is quite good and allows me to crank most CDs to full-volume without any clipping of the tubes.

I have kept the tube volume control permanently at 12 o'clock position. The clipping issues happen only if i crank the tube volume control to around 3 o' clock position.

 

[Hari Iyer]

What happens now when you crank up the vol pot to 3 o clock position?

Frankly I don't see the need for a linestage. What is the input sensitivity of this amp? My little experience with these things says that the lack of SPL is not due to input but at the output, e.i, the speakers, listening environment etc. This assumes you are using a CD player as source.

Secondly, I don't know what kind of driver circuit and OPT this amp is using. Its easy to clip 300B if the driver topology is not robust. Most budget Chinese 300B schematic I have seen take a lean approach on this. Left to me I would gut out this amp and rebuild with a different driver tube/topology.

Just my humble thoughts. I really wouldn't dare to cross swords with a celebrated designer like you.

I have tried using the 300Bs without the pre-amp and will say that it struggles without the pre-amp as the sensitivity of its input stage is not good enough to achieve loud listening level. Infact without the pre-amp any cranking of the volume above 12 o'clock cause it to clip and cause listening fatigue.

With the pre-amp i could add headroom to the levels and the tube now sings well with it. Any one going for the 300Bs should always consider the pre-amp too else will be disappointed. I have seen and heard a very good JfET input pre-amp build by Omishra some years ago.

 

[omishra]

Hari, that preamp was pass B1. Now i moved to direct coupled version of it. No capacitors in signal path. Now its open sounding to max extent.

 

[jls001]

Hari, when you have time and inclination, do build a direct coupled B1. It is a very good buffer. Of course this assumes that your power amp can be driven directly by the voltage from your sources, since DCB1 has unity gain but does come with volume control.

 

[Hari Iyer]

did not find any ckt diagram easily for the direct coupled pass B1 in the net.