Pre amp therapy / therapy pre

[abbey road d enfer]

I tried to measure the primary inductance but couldn't obtain consistent results with the resonant circuit technique described here :
- Series cap = 10nF-> peak at 900Hz-> 3.13H
- Series cap = 100nF -> peak at 160Hz -> 9.65H
- Series cap = 1uF -> peak at 30Hz -> 28.14 H

How should I interpret those results ?

That just shows the large dependance of nominal inductance of iron-core inductors against frequency. You may observe varaiations against level, too. That's why it is important to measure such inductors in their operating conditions. When you want to assess a xfmr for LF response, you have to measure it at LF. Typical LCR meters make measurements at 100 and 1k, that is not enough for correct assessment; inductance at 20Hz (where you want LF extension) is probably 3x that measured at 100Hz. But the operating level of these LCR meters is so small it creates an error as large again.

 

[abbey road d enfer]

- I've used the "Jensen" procedure to determine the optimal load, and finally the best load is 9k1 + 470pF

That's exactly the conclusion I came to, using your measurements. So why do LL recommend a set of values that cannot deliver acceptable performance? Typo? Did they answer your questions?

Final conclusion : you can use a LL1528 for this project, but it's not the ideal choice.

Exactly. Now, what would it be good for? I can't find a combination that gives satisfactory results...

 

[ricothetroll]

Thanx for your answers !

I must say I've been pretty disappointed by those mic trafos, I bought them cheap a few years ago, trusting the brand. When I pointed that this 3nF value could be an error, Per anwered :

"Quite possible!
Or the source impedance I used was much less...
I must admit I do not remember."

Well ended up thinking the probability that I ever use a mic which impedence exceeds 300R is very low, and even then I could use another preamp. And if someday I have 600€ to spend, I could get the Sowter in / Lundahl out option that Jonte used fo his unit.

 

[digitall2000]

Hi,

I finally decided to make a PCB for this project, in order to be able to use the Lundahl LL1528 I have in stock, and also to use relays for some switching, to make the audio path as short as possible. Note that the input switching  (phase, PAD...) are done externally. The tube sockets are ceramic that go through the PCB and are wired from the bottom. That's in order to be able to fit the PCB in a 2U rack, else the height would be excessive.

Any opinion on the routing ?

For now I have used Eagle's "polygon" function to make the ground trave as beefy as possible. Would it be a good idea to fill all blanks on the PCB with ground ?

Thanx in advance for your advices.

Best regards.

Eric

Hi Eric,

Would be so kind as to share your eagle files?
I would like to burn a board here at home

thanks,

Lance

 

[ricothetroll]

Here are the Eagle's files.

https://drive.google.com/file/d/1UD_CHM5Sgg1YBZsKpL85vAj141968CL4/view?usp=sharing

Please keep those things in mind about this board, it's not that perfect :

- It uses LL1528 trafos, that aren't really good at this particular job (are those bstrds good at any job ?). Modify the board for your input trafo.

- It uses HUGE russian paper in oil caps. If you plan to use regular film caps you could spare some room and fit the circuit on a regular 100x160mm board

- See attached image to see how the tube sockets are wired to the board. That's for being able to use a 2U rack. If you have a 3U it might be a good idea to use it and put some normal PCB tube sockets (modify the board accordingly).

- Put a proper Molex-like connector for 6.3VAC heaters. I used a 12.6VAC trafo, that's the reason why there's those single pads.

That's what comes to my mind looking back at this circuit, which otherwise works pretty fine. With a few Eagle skills you can make it better. I'll route a better version of it when I'll have some free time.

Best regards.

Eric

 

[digitall2000]

Very Cool
thank you very much
lance

oh yeah, not to be a pest but, i am fairly new to eagle and even after turning on all the layers in board view and changing the background to white i don't see the ground plane.  any ideas?
thanks again for sharing

 

[syn]

ratsnest

 

[digitall2000]

thank  you

 

[digitall2000]

hi Eric
hey would mind posting a picture of the front of yer theraPre?
thanks,
lance

 

[digitall2000]

Here is a project which started as a thought piece for a colored pre for a friend and eventually developed into a therapy project. I just wanted to build something completely different from my product line and have fun. Let me say first that I'm not particularly experienced in p2p wiring and this project is probably not going to be commercial and does not reflect my general building practices. That said, look at the (bad) pictures.

http://picasaweb.google.fi/jonte.knif/TherapyPre#5443655352963125618


-Jonte

hi Jonte,
this is a great project, especially if one is try to learn a few things, like me , from people that are well versed in the art of design and layout.
i am hoping that you and any one else that has done a ptp of this preamp  would repost the pics of their work because the links to the original pics seem to be dead
thank you ever so much,
lance

 

[ricothetroll]

Here is the P2P layout I did but didn't build. It's inspired by a picture of Jonte's preamp.

 

[digitall2000]

thank you

 

[ricothetroll]

I forgot I asked Jonte if I could post his photo, which he agreed. Here it is !

 

[Murdock]

Thats great, thanks!
Do you by chance also have a picture of the frontpanel?

 

[ricothetroll]

[edit] tried to post some more but the site returns an error "your file failed securyty checks". Not the first time, still do not get why. Here are some other pics from the inside of the preamp :
https://drive.google.com/file/d/147rC96rv98_2bVghWY2IepaSzJIq5PJc/view?usp=sharing
https://drive.google.com/file/d/18nEFSaxm98DvmNqHSv_GJLEQP2LfTh9s/view?usp=sharing
https://drive.google.com/file/d/1So5AVssTJmzWnmzoIoPN_Jc1e4fBqgT5/view?usp=sharing
Here is one of the front panel :
https://drive.google.com/file/d/1WIBFtCaWsnwpMuoil4dYvlwUi0SSZUGi/view?usp=sharing

 

[Murdock]

Here is one of the front panel

hmm, can't see the picture... says "404 - Attachment Not Found".

 

[ricothetroll]

hmm, can't see the picture... says "404 - Attachment Not Found".

previous post edited, pics stored on google drive (looks like something is wrong with the site today)

BTW I intend to label it, but I'm still waiting for the ink stamps I intend to use for it...

 

[Murdock]

Sorry for reviving this old thread, but I thought it's better than making a new one...

I have two Rohde & Schwarz output transformers which are gapped and have a 12:1 ratio. And I thought I could maybe use them for this build.
The thing that I'm unsure about is the center tap which is not really a center tap in those transformers.
One winding has a dc resistance of 3850 Ohm and 28.9H inductance at 100Hz and the other winding has 5330 Ohm and 30.3H.
How would that imbalance affect the performance of this preamp? Could I counteract that imbalance with modifying the two 10k resistors on the primary windings?

 

[abbey road d enfer]

DC resistance and nominal inductance are less important than balance. The winding with higher DCR and inductance is probably the one that's wound over the other (less close to the core), but I would bet they have both the same turn ratio with the secondary.
If I was retentive about balance, I may add a 1.5 k resistor on one side, but I doubt it has any noticeable consequence.