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First stage phono preamp transformer output coupled

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Posted on February 13, 2017 at 00:51:02
d888sp4@gmail.com
Audiophile

Posts: 11
Location: Italy
Joined: January 31, 2006
Hi,
I am planning a phono preamp where input stage is a E810F triode connected (Rp=1000ohms) transformer coupled to the RIAA LCR 600 ohms network.
The valve is biased at 30mA.
I would use a step down transformer (4:1 or similar ...) for best impedance matching with RIAA network.
What transformer do you recommend that supports 30mA current?
LL1671 is the right choice or is there something better?
Thanks for your help.
Alberto

 

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RE: First stage phono preamp transformer output coupled, posted on February 13, 2017 at 04:41:16
KevinC
Manufacturer

Posts: 2906
Location: NC
Joined: April 19, 2001
The LL1671/30mA is the best choice for the application you describe.


Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: First stage phono preamp transformer output coupled, posted on March 6, 2017 at 08:43:27
d888sp4@gmail.com
Audiophile

Posts: 11
Location: Italy
Joined: January 31, 2006



Hi Kevin,
I tried to simulate with LTspice the input stage with E810F.
With resistive RL=10k provide excellent performances, however with LL1671 4:1 coupled to output load of 600ohm (lcr riaa network), appears that the low frequency response suffers an attenuation from 100 Hz...
I suppose that 600ohm on the secondary is about 10k reflected on the primary.
I wrong anything or do you think there is some other more suitable transformer to use?
I tried to replace it with the LL1692 and everything in spice seems ok, but obviuosly this may not work with 30mA bias...
Thank you.
Alberto.

 

RE: First stage phono preamp transformer output coupled, posted on March 6, 2017 at 16:54:32
KevinC
Manufacturer

Posts: 2906
Location: NC
Joined: April 19, 2001
It is difficult to make out the schematic values and the graph axis on the forum post. What operating point are you using for the E810F, 30mA? My experiments with 7788s have given me Rp=4K @ Ip = 12.5mA and Va = 155VDC. Of course, at higher currents, the Rp will be lower.. However, Amperex 7788s are, as a rule, too microphonic for phono use. I chose to use the 6J52P, which was quieter, albeit with an 18% higher Rp at 12.5mA. Although that circuit was a differential output stage using the LL1689/PP, the LL1692A/30mA (Lp = 54H @ 3.5:1) would be a better choice for your SE circuit. Perhaps the LL1971 just didn't have enough primary inductance.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: First stage phono preamp transformer output coupled, posted on March 7, 2017 at 00:27:30
d888sp4@gmail.com
Audiophile

Posts: 11
Location: Italy
Joined: January 31, 2006
Hi Kevin,
in my design 7788 is triode strapped, bias is around Va=140V, Ip=30mA, with Rp=10k to get max transconductance (50mA/V) that gives a total gain around 40 (32dB).
LCR RIAA network 600ohm is the load on secondary trasformer. With 4:1 transformer it is reflect to primary around 10k that match 7788 Rp needed.
My reasoning is that correct?

LL1692A/30mA version is in stock or is a special order version ?
What do you think about LL1660/30mA version 4.5:1 connection (100H) in my SE ?
Thanks.
Best regards.

 

RE: First stage phono preamp transformer output coupled, posted on March 8, 2017 at 17:34:19
KevinC
Manufacturer

Posts: 2906
Location: NC
Joined: April 19, 2001
If you use a plate resistor of 10K, the plate impedance is about the inverse of 1/10K+1/4K or 2.86K. Formally speaking the plate resistor and the tube are in parallel, so the parallel combination of their impedances are the gain stage impedance. If you use a transformer to load the 7788, then the stage impedance is that of the 7788 transformed by the turns ratio of the transformer squared. So if you assume the 7788 has a plate impedance of 4K and you use an LL1692A/30mA with a turns ratio of 3.5:1, then the output impedance of you stage is 326 ohms. The use of a 600ohm load resistor is unnecessary with this set-up, but we should try to get closer to 600ohms with the 7788 and the transformer. Using the LL1660/30mA at 4.5:2 gives about 790ohms, which is closer. To get closer yet will require adjusting the operating conditions of the 7788.

If I haven't lost you in all of this, you can see the process. If you are lost, well let's chat more.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: First stage phono preamp transformer output coupled, posted on March 8, 2017 at 23:57:32
d888sp4@gmail.com
Audiophile

Posts: 11
Location: Italy
Joined: January 31, 2006
I can not fully understand your explanations, so in order to not make you lose too much time chatting, I sent you an email with the circuit design (ltspice).
In this way you can well understand what I'm trying to build...

Thanks.
Carlo Alberto.

 

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