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MP3 phono cascode mod

152.208.100.207
Posted on December 14, 2022 at 03:52:37
MP
Audiophile

Posts: 44
Joined: July 2, 2001
 I am currently using old Grado Reference (5mV) cartridge. Volume level is at 8-9 o'clock.

Some old treads mentioned cascode mode in MP3 phono stage: removing middle two 12AT7 tubes, and placing jumpers (between pins 1-3 and pins 6-8) in tube sockets to reduce gain and better sound.

Any experience or suggestions?

Does factory have jumper/adapter for purchase?

Thanks.

 

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RE: MP3 phono cascode mod, posted on December 16, 2022 at 08:02:01
MP
Audiophile

Posts: 44
Joined: July 2, 2001
 After re-read the old threads, I did the "Cascode de-activation mod) - removed the middle two 12AT7 and placed 9-pin adaptors with jumper wires soldered in.

The phono stage is much quieter, no tube rush with volume setting up to 4 O'clock.

Sound is more transparent with bigger stage. Playing at volume level is at 12-1 O'clock. Much happier.

 

Does factory have jumper/adapter for purchase? Yes. nt, posted on December 16, 2022 at 12:43:45
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 -

 

RE: Thank you, Ralph. New setup with SUT, posted on December 20, 2022 at 07:54:33
MP
Audiophile

Posts: 44
Joined: July 2, 2001
 Built an external SUT (Hashimoto HM-7) last week. Balanced connection.

At x15 (23db gain), Benz Ruby H (0.6 mV) plays at 11-12 O'clock with Cascode de-activation Mod and at 8 - 9 O'clock with stock Cascode. Like the Cascode de-activation Mod more.

Without SUT, Benz Ruby H plays at 1-2 O'clock.

The big different between SUT and without SUT is that with SUT there is quieter background, and music could be played louder (than stock setup) without triggering temporary speaker shutdown (QUAD 989).

It has been a lot of fun playing with different settings.

What is the phone stage gain with cascode deactivated?

 

Variables, posted on December 20, 2022 at 09:38:34
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 The phono gain without the cascode is about 45 dB. Works nice with 5mV.

Pay attention to loading of the output of the SUT. If its designed for your cartridge the stock load of the preamp (47K) is probably fine. But if its not intended specifically for your cartridge, there may be additional loading needed. The loading strip on the MP-3 might be handy for this. If you change cartridges, the loading of the transformer will change with it. If you are running an SUT like this (for example, a Jensen) the loading will be a resistor and capacitor network (Jensen has suggested values for all known cartridges on their website).

 

RE: Variables, posted on December 20, 2022 at 10:16:35
MP
Audiophile

Posts: 44
Joined: July 2, 2001
 Ralph, thank you for the info.

The SUTs recommend 47K phono loading.

Will look into the Jensen's loading network info.

 

RE: Variables, posted on December 20, 2022 at 10:19:08
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 That sounds to me that the SUT is designed for your cartridge. Is that correct?

If not, then it will take some testing on the bench with your cartridge to sort out what the correct loading should be.

 

RE: Variables, posted on December 20, 2022 at 11:14:00
MP
Audiophile

Posts: 44
Joined: July 2, 2001
 They are generic off the shelve SUT, not specifically designed for my cartridge. My cartridge has fairly high internal impedance (90 ohms).

"If not, then it will take some testing on the bench with your cartridge to sort out what the correct loading should be". What kind of bench testing is needed?

I am under impression that the primary load cartridge sees is (SUT provides = 47K/SUT run ratio square), ideally is 7-8 times of cartridge internal impedance. If the primary load cartridge sees is less, like 3-4 times, efficiency would be lower - less gain.

Read it somewhere that one could increase phono stage impedance from 47K to 100K or higher to give higher primary load that cartridge sees and increase efficiency (gain). Do I understand correctly?

 

RE: Variables, posted on December 20, 2022 at 11:26:45
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 Read it somewhere that one could increase phono stage impedance from 47K to 100K or higher to give higher primary load that cartridge sees and increase efficiency (gain). Do I understand correctly?

I don't think so. You have to load the transformer correctly in order for it to express its turns ratio correctly. If the load value is too high, the inter-winding capacitance starts to play a role and the frequency response won't be as flat.

I would check with the manufacturer and see if they have a recommended network for loading with your cartridge. Don't forget that the capacitance of the phono cables also plays a role- for this reason the output cable should be as low capacitance as possible!

 

RE: Ralph, thank you. Very informative. NT, posted on December 20, 2022 at 13:33:11
MP
Audiophile

Posts: 44
Joined: July 2, 2001
 NT

 

RE: Variables, posted on May 5, 2023 at 04:30:19
MP
Audiophile

Posts: 44
Joined: July 2, 2001
 Installed a cartridge Dynavector Te Kaitora Rua (0.26mv, 6ohms). SUT Loads at 30X. Very happy with the sound.

I think my MP3/M60/Quad 989 likes high phono input (7.8 mV).

Bought an used Coincident Total Eclipse (94 sensitivity, 14 ohm impedance), waiting for delivery. Read old posts that M60 and total Eclipse are great match.

 

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