Hello all,
First of all, I have to say that my knowledge about electronics it's very limited. And it's the first diy audio project that I have ever made.
Some months ago I built a stereo tube preamp that I use for bass guitar. It's a modified Alembic F-2B.
Some people, and even Alembic, say that the preamp does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity. And I found that some people was adding a cathode follower to lower the output impedance (40 kOhm in the original unit).
All I made is to copy the schematics from the following link (I think it's not working anymore).
403 Forbidden
As I mentioned before, he added a white cathode follower and a line level output transformer to get balanced line level ouputs.
I attach the schematic that I made based on his project. I only added a 4PDT swith for stereo/mono mode.
The unit is working well and sounds good. But I am wondering if I could improve it by upgrading the output transformers. I used Edcor WSM 15K600 line output transformers (5:1 ratio). These transformers are quite cheap and maybe a better quality transformer will improve the sound.
I have read that Jensen make very good (and expensive) transformers. Looking in their website I saw that they make a transformer for vacuum tube line outputs: the JT-10K61-1M. But it has a 4:1 ratio wich is different from what I have now. So I don't know if this could fit well in my preamp.
Do you think that a better transformer wil improve the preamp? Do you think the jensen T-10K61-1M will work better than what I have now? Any other transformers that I should consider?
Thanks for your help,
Unai
First of all, I have to say that my knowledge about electronics it's very limited. And it's the first diy audio project that I have ever made.
Some months ago I built a stereo tube preamp that I use for bass guitar. It's a modified Alembic F-2B.
Some people, and even Alembic, say that the preamp does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity. And I found that some people was adding a cathode follower to lower the output impedance (40 kOhm in the original unit).
All I made is to copy the schematics from the following link (I think it's not working anymore).
403 Forbidden
As I mentioned before, he added a white cathode follower and a line level output transformer to get balanced line level ouputs.
I attach the schematic that I made based on his project. I only added a 4PDT swith for stereo/mono mode.
The unit is working well and sounds good. But I am wondering if I could improve it by upgrading the output transformers. I used Edcor WSM 15K600 line output transformers (5:1 ratio). These transformers are quite cheap and maybe a better quality transformer will improve the sound.
I have read that Jensen make very good (and expensive) transformers. Looking in their website I saw that they make a transformer for vacuum tube line outputs: the JT-10K61-1M. But it has a 4:1 ratio wich is different from what I have now. So I don't know if this could fit well in my preamp.
Do you think that a better transformer wil improve the preamp? Do you think the jensen T-10K61-1M will work better than what I have now? Any other transformers that I should consider?
Thanks for your help,
Unai
The WSM are good transformers; better than many transformers (from days when we used transformers in audio). The Jensen is of course better, but they charge for it, and very few gigs would reveal any difference.
I have used power transformers here. Yes, I had to be careful about nearby hum sources, and select my parts for over-5KHz response (few stage instruments have any useful output >5kHz). One summer I had a string of top acts and nobody said "I hear a cheap transformer!"
5:1 or 4:1 is surely within the range of your Volume control.
I have used power transformers here. Yes, I had to be careful about nearby hum sources, and select my parts for over-5KHz response (few stage instruments have any useful output >5kHz). One summer I had a string of top acts and nobody said "I hear a cheap transformer!"
5:1 or 4:1 is surely within the range of your Volume control.
Thank you, PRR.
Good to know that 4:1 can also work for my preamp.
As it is a two channel unit I am planning to change one of the transformers so I can really A/B them. Because sometimes it's not easy to say how something used to sound like. I will post my impressions once I make the tests.
Thanks again.
Good to know that 4:1 can also work for my preamp.
As it is a two channel unit I am planning to change one of the transformers so I can really A/B them. Because sometimes it's not easy to say how something used to sound like. I will post my impressions once I make the tests.
Thanks again.
I was going to suggest Hammond 1140-LU-A as a possible substitute but looking at prices online they seem to be going around same as the Jensen series. Lundahl is also very good, but can get expensive. Sowter 1160 series as well known for studio equipment. Very nice build on your preamp BTW, clean layout! I have not used the Edcor but have heard from others that they are well made and excellent price/value. Please let us know how your comparison goes, thanks for sharing pictures of your project, it looks great.
Hello again,
I considered the ones mentioned by Shanx but for the same price range I ordered the Jensen jt-10k61-1m transformer and assembled it in the preamp. Now I have the Edcor WSM 15k/600 in the channel A and the Jensen in Channel B. Both channels are the same. The only diffrence is the output transformer and also the capacitor that goes before the transformer. The Jensen came with it's own capacitor so I decided to install it thinking that it would be the best match. The original capacitors were 2,2 µF (I think they are polyester). The one that came with the Jensen it`s a 22 µF electrolytic cap.
I attach some pictures.
I recorded a short bass riff in the looper and then I recorded it through channel A and channel B. I also recorded the loop through a Darkglass B7K DI. Just to have another reference.
I attach the clip (I had to put it in a zip folder). The order is the following one:
loop 1: Darkglass B7K DI
loop 2: Channel A (Edcor WSM 15K/600)
loop 3: Channel B (Jensen jt-10k61-1m)
Honestly, I think the difference is barely audible (at least for my ears).
It was interesting to make the test but I think the upgrade it's not worth it. The Edcor already sounds great.
Let me know your opinion.
Unai
I considered the ones mentioned by Shanx but for the same price range I ordered the Jensen jt-10k61-1m transformer and assembled it in the preamp. Now I have the Edcor WSM 15k/600 in the channel A and the Jensen in Channel B. Both channels are the same. The only diffrence is the output transformer and also the capacitor that goes before the transformer. The Jensen came with it's own capacitor so I decided to install it thinking that it would be the best match. The original capacitors were 2,2 µF (I think they are polyester). The one that came with the Jensen it`s a 22 µF electrolytic cap.
I attach some pictures.
I recorded a short bass riff in the looper and then I recorded it through channel A and channel B. I also recorded the loop through a Darkglass B7K DI. Just to have another reference.
I attach the clip (I had to put it in a zip folder). The order is the following one:
loop 1: Darkglass B7K DI
loop 2: Channel A (Edcor WSM 15K/600)
loop 3: Channel B (Jensen jt-10k61-1m)
Honestly, I think the difference is barely audible (at least for my ears).
It was interesting to make the test but I think the upgrade it's not worth it. The Edcor already sounds great.
Let me know your opinion.
Unai
Attachments
Hi Unai, firstly the build project came out great and congratulations! I had a listen of the sound files, and honestly couldn't hear any major difference from Edcor to Jensen. Both recordings sounded really good.
The Darkglass B7K DI to my ears sounded a little less bright on the high frequencies, but that just may be the settings on it.
Cheers!
The Darkglass B7K DI to my ears sounded a little less bright on the high frequencies, but that just may be the settings on it.
Cheers!
One think I like about using output transformers, it allows for electrically isolated output signal from the preamp circuit ground. This can eliminate hum and ground loop potentials. You could for example plug the A channel into an amp, and run the Bchannel to a mixing desk, and they shouldn't interfere with each other.
Also another layer of safety from the B+ rail of the tube circuit..
Also another layer of safety from the B+ rail of the tube circuit..
Hello dra9his,
I just read your post. Sorry for the late answer.
I used a toroidal transformer from tubetown website. It's a 47VA transformer with 250, 6.3 and 12.6 secondary windings. The primary is 220, 230, 240. Here the link: https://www.tube-town.net/ttstore/en/toroidal-47va.html.
I am afraid it wont't work for 110 mains. But there will be other options for sure.
Thanks everybody for the comments. I am using the preamp for home recording and it sounds great.
Unai