Phono Stage tube PCF802 from VLAD ~ any info?

[Tomcat1969]

Every day is a school day at Lenco Heaven. Many, many thanks guys.

[Taku]

  This leaves the cathode with no signal and thus no negative feedback and full gain.

Could the absense of negative feedback be part of the reason people, me included, have hoped for some more bass from this thing? Well, I really need to remove the capacitors and listen, then I know.

[Tomcat1969]

I have removed the C5 caps and I'm listening to the first LP...  Jazz.

Really, I should just get some Cypress Hill on the tt.

Anyway, at the moment, it's sounding very good.

[Tomcat1969]

Arndt. You made a good point about the cartridge/stylus.

I'm using an 881S with an Ogura line contact. Perhaps a little dry. I'll try an XV-15 with a D-II-S over the weekend.

[niclaspa]

Could the absense of negative feedback be part of the reason people, me included, have hoped for some more bass from this thing?

No, that will not change the frequency response.

A possible reason for poor bass is if the following amplifier has low input impedance.  As explained in this post https://www.lencoheaven.net/forum/index.php?topic=37049.msg482444#msg482444 the phono stage's output impedance is 17 k < 100 Hz.  If the following amplifier has 25 k in input impedance, the bass will be reduced with 40%.  If it has 50 k the reduction will be 25%.  In such case, a buffer, e.g., in the form of a cathode follower solves the problem.

I have a line stage with large input impedance.  The bass is excellent.  I have also measured the frequency response with an anti-RIAA filter and the response was flat at least down to 30 Hz.

[Kno]

…..  Jazz.

LOL

[wenig watt]

Hi,
My friend Bernd, who develops tube amps and usually has something to complain about, was noticeably impressed by the circuit. Very precise equalisation and very good bandwidth.
If you need more somewhere, you can make it happen. My point was that the characteristics such as increased transparency through the silver mica can be too much in a brightly tuned system. My new push-pull power amp has much more bass capability, but also a much more extended treble range than my SE power amp.
25 Hz linear is no problem in my system, but an open baffle doesn't make a boomy bass. If now also linear up to 20 kHz are reproduced, that's too much for me. I don't have much padding or other high frequency insulation in the room and prefer a slightly falling high frequency range. Nevertheless, the sound of the silver mica is clearer and remains clearer and more natural even with a lowered high frequency range.


Best regards

Arndt

[Tomcat1969]

Aha! Input impedance.

Thanks again Niclas. I had read the post you linked to but didn't remember that my following preamp has 25k input impedance set by shunt resistors before the stepped attenuator volume control.

I'll get them changed today and report back.

[Taku]

No, that will not change the frequency response.

A possible reason for poor bass is if the following amplifier has low input impedance.  As explained in this post https://www.lencoheaven.net/forum/index.php?topic=37049.msg482444#msg482444 the phono stage's output impedance is 17 k < 100 Hz.  If the following amplifier has 25 k in input impedance, the bass will be reduced with 40%.  If it has 50 k the reduction will be 25%.  In such case, a buffer, e.g., in the form of a cathode follower solves the problem.

I have a line stage with large input impedance.  The bass is excellent.  I have also measured the frequency response with an anti-RIAA filter and the response was flat at least down to 30 Hz.

Thank you for taking your time to make this clear Niclas. Another school day . My power amp looses bass when negative feedback is disconnected. That was the reason for my question.

Just in case some one who has not heard Vlad's pre is reading this thread and thinks I'm saying the bass of the pre is poor I want to stress that that is not the case. It's wonderful! As Niclas has measured here https://www.lencoheaven.net/forum/index.php?topic=37049.msg483596#msg483596 the output impedance at 20 Hz is 13.5 kohm. My power amp input is 100kohm so it's about just enough. It's nice that I can add a buffer, although I don't really need it. My previous pre was some kind of bass monster and it's output impedance was 1 kohm. I clearly heard the change when I switched to Vlad's pre but that change made me happy  .

[Tomcat1969]

Good point Tuukka.

This a fantastic phono stage, no doubt.

When I first contacted Vlad and described my 834 clone (built with the guide found here at LH) to him Vlad, to his credit, thought that perhaps my 834 would be superior and said so.

It's not... I'm glad I decided to get my hands on this phono stage.

[Tomcat1969]

I replaced the shunt resistors in my pre. 25k changed to 82k. It seemed to have the effect of increasing the bass but I felt the sound was too dark so I replaced them again this time with 68k resistors (kiwame) and I think I'm satisfied with the sound now.

Is this a legitimate technique? My knowledge of actual electronics theory is extremely thin I would say.

[niclaspa]

I replaced the shunt resistors in my pre. 25k changed to 82k. It seemed to have the effect of increasing the bass but I felt the sound was too dark so I replaced them again this time with 68k resistors (kiwame) and I think I'm satisfied with the sound now.

Is this a legitimate technique? My knowledge of actual electronics theory is extremely thin I would say.

I am not sure if you can change the shunt resistors on your preamplifier without adverse effects.  That depends on the circuit.  I think that a buffer would solve the problem or using another preamplifier.

If you want a buffer, you could use the schematic at the bottom of this page: http://www.valvewizard.co.uk/accf.html  I am sure that the high tension power supply of the phono stage can feed this buffer as well. You would need another transformer for the heater supply to the ECC82.

[Tomcat1969]

Thank you Niclas. I'll give it a go. I've got a transformer and a valve etc.

[Tomcat1969]

A silly question?

Niclas. Do I build the buffer as 1 unit or double up the components and build 2, one for each channel?

[niclaspa]

You need one unit per channel.  Note that the tube contains two triodes.  Therefore, you only need one tube for both channels.

You should insert the buffer at the points marked with arrows:



Break the connections to C11 and C12.  The input of the buffer should be connected to the left side.  The output of the buffer should be connected to C11 and C12.  Input and output marked with arrows here: