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Old 3rd June 2015, 09:55 AM   #411
audiodesign is offline audiodesign  Italy
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Send a message via MSN to audiodesign OLD THREAD Hi-end phono preamplifier by Andrea Ciuffoli
Artur5,
Tubes sound better with high voltage and distortion is lower with high voltage.

You can work also with 20V on anode but the result is not the same.
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Old 3rd June 2015, 05:42 PM   #412
Artur5 is offline Artur5
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Quote:
Originally Posted by audiodesign View Post
Artur5,
Tubes sound better with high voltage and distortion is lower with high voltage.
You can work also with 20V on anode but the result is not the same.
Andrea, if you read well my post, I wasn't saying to put less voltage on the anode. I was saying to keep the same voltage and current, lowering the B+ tension and the value of the plate resistors in order to spare wasted heat in the mentioned resistors and stress less the PSU components.
As far as the tube is concerned, it would work on the same electrical conditions ( same tension on anode and cathode, same standing current )
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Old 31st August 2015, 12:13 PM   #413
tangmonster is offline tangmonster  South Africa
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Quote:
Originally Posted by hifiwolf View Post
Think they are not enough. If I am comparing to high end transistor amplifier –there are more powerful sub- bass.

Please tell about your experiences with this amplifier.
Hi There

I have been using mine off an on for many years. I Also felt that the bass was a bit low from my turntable so recently I revisited the design and actually put it on a Function generator and an oscilloscope to measure the RIAA curve.

Sadly what i found was that at 50hz the amplfication was 4db down from what it should be according to the RIAA curve.

The simple truth is that the cathode resistor (50ohm) and a 270uF capacitor in parralel with it attenuates <100hz frequencies too much.

I experimented with different resistor sizes and larger capacitors (up to 1000uf) all of them pushing up bass responce. But in the end settles on using a Infrared led in place of the 50ohm and 270uF cap.

I like the idea of removing the Capacitor. The LED changes the cathode voltage up the 0.5V cathode voltage to 1.15V

BUT

There is SO much more bass! And using the function generator and oscilloscope i measure correct RIAA curve to within 1db.

With the 22K resistor and 210Vdc B+ I measured 105V accross resistor wchich calcualtes to just less than 10mA bias through valve.


I was very close to changing 22K resistor to 10mA current source but decided to rather use with 22K and LED for the time being.

Very similar circuit for D3a found here:

AudioRoundTable.com: Group Build => RIAA preamp project


It is clear from the circuit diagram that JOGI post that bass responce will be MUCH better due to using 390 ohm with 470uF on cathode.

50 ohm with 270uF attenuates bass FAR to much.

Last edited by tangmonster; 31st August 2015 at 12:18 PM.
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Old 31st August 2015, 07:51 PM   #414
nikhilji is offline nikhilji  United States
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Cool, Thanks for the tip @tangmonster. Will definitely have to try this tweak.
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Old 31st August 2015, 09:31 PM   #415
Artur5 is offline Artur5
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Sorry @tangmonster, but biasing the D3a with 9.5mA is not the best choice IMHO. The original design is biased aprox. at 12mA, which will give less distorsion, although I'd preferred 14-15 mA for better sound.
Besides, it makes little sense that the RIAA error has much to do with the cathode resistor/capacitor. The tube is not in the RIAA network. The bass drop for a 50ohm/270ufarad combo on the cathode should be very small at 50 Hz -1dB maybe.
The design you mention uses the D3a in a very different configuration : grid 3 to anode instead of cathode, grid 2 to anode trough a 100R resistor vs. directly to anode and also uses higher bias current ( 11mA. )

Last edited by Artur5; 31st August 2015 at 09:37 PM.
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Old 1st September 2015, 07:39 AM   #416
tangmonster is offline tangmonster  South Africa
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biasing the D3a with 9.5mA is not the best choice IMHO. The original design is biased aprox. at 12mA, which will give less distorsion, although I'd preferred 14-15 mA for better sound.

I agree, My plan is to use it for a while and possibly lower anode resistor to make bias current higher. I measured 0.55V over the original 50ohm resistor which calculated to 11mA. I agree if i look at the d3a pdf load lines that 15 looks more linear.


it makes little sense that the RIAA error has much to do with the cathode resistor/capacitor. The tube is not in the RIAA network.

Again i have to agree, The 270uf cap has 0% to do with the RIAA network, BUT it has VERY much an effect of Valve gain due to adding negative feedback at lower frequencies.

If you calculate the resistance of 50ohm and 270uF in parralel at various frequencies is:

1khz = 0.007ohm
100hz = 0.7ohm
50hz = 2.6ohm
20hz = 12.9ohm

2.6ohm and 12ohm is a large persentage of 50ohm and you can clearly see in the math how it will have an effect on bias/gain at these frequencies.

The higher resistance at lower frequencies WILL give feedback to the cathode voltage which WILL have an effect on gain.

I measured a somewhat large effect. I upped the capacitor to 1000uf which helped with lower frequencies but not enough.


The design you mention uses the D3a in a very different configuration

Again I agree. I was simply refering to the 470uf in parralel with 390 ohm which gives much lower impedance to low frequency than 50ohm in parralel with 270uf.

Last edited by tangmonster; 1st September 2015 at 07:51 AM.
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Old 9th March 2016, 10:29 PM   #417
JOIMONF is offline JOIMONF
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Hello Jogi, a little late but now I try to complete this project. I have the boards populated with anything than tube shockets after these years. Following your advice, I don't use the matching power supply board supplied, instead, I have SSHV2 stored for too much time. Also I have AZ1 with mesh plates. please lets ask for advice, what inductance value on two chokes? and also don't know what value in three capacitors (I have spare 10 and 20 henry ASC oil cans.)
Also I need order the power transformer. Please lets me know what voltage in secondary B+ to power such a power supply.
Thanks in advance, Jordi.
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Old 15th October 2016, 11:42 PM   #418
apelizzo is offline apelizzo  Australia
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After long time this project was started I am completing mine. I just noticed that the D3a triode strapped is unstable and oscillates badly. As soon as I connect the output capacitor it goes mad. I noticed that the screen grid is hard wired to the anode without a resistor. Could be that the cause? Did anyone have this issue?
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Old 2nd September 2018, 09:21 PM   #419
Jezza56 is offline Jezza56  Australia
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Default Revisiting D3A Phono

Hi,
I would like to know if anyone is still visiting this forum, I have the phono stage that I put together about 6 years ago but had no success in getting up and running. I then took ill for some time but now I would like to try and get it up and running. I must admit I donot have much experience in electronics, relying basically on what my father passed on to me 50 years ago..lol hope someone can come to my rescue and steer me in the right direction.

Regards

Jerry.
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Old 2nd September 2018, 09:31 PM   #420
rayma is offline rayma  United States
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Quote:
Originally Posted by Jezza56 View Post
I have the phono stage that I put together about 6 years ago but had
no success in getting up and running.
Ok, post the schematic and some photos. What have you done to check the circuit,
and what test equipment do you have?
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