My DIY Pre/phono Pre/Active XO/tube buffer

engtaz

looking good Ben. Keep in up.

engtaz

ben62670

Thanks guys. Today I got tired of waiting for the caps to burn in. There were two sets of DC blocking caps. One set was on the input stage, and one was on the outputs of the gain stage. Having the active XO's kills the need for an DC blocking in the circuit. Well I have my bass slam back, and the highs are very detailed. I also manipulated the gain stage for +6d do to the Adcom's requirement for a high voltage signal. I am now actually hearing more detail in my music than ever before. There is about 60hrs on it now. BTW the phono stage is still DC protected;)
Ben

Systems

ben62670 wrote: »

There were two sets of DC blocking caps. One set was on the input stage, and one was on the outputs of the gain stage. Having the active XO's kills the need for an DC blocking in the circuit.

Lookin good Ben.;)
Those 2134's are low offset devices so removing the caps should present no problems as long as the source doesn't dump 10's of millivolts of DC.

ben62670

GV#27 wrote: »

Lookin good Ben.;)
Those 2134's are low offset devices so removing the caps should present no problems as long as the source doesn't dump 10's of millivolts of DC.

I am keeping the caps on the board for the full range output jacks, but the XO boards do not need DC blocking. So far the hardest part to find is a 6pdt on/on switch for line level. I want to be able to bypass the gain stages for the passive pre option. My digikey catalog should be here soon.
Ben

Systems

really,it needs 6 poles? I doudt you will find a toggle with that many.

ben62670

L/R/G in L/R/G out. This way I can completely bypass all active circuits with a flip of the switch;) I will also need another for the tube buffer. I may just use the buffer on the highs. Right now my XO point is 2000.
Ben

ben62670

I am beating the crap out of this one. If it fails it will fail here;) As for the case I am getting a full case in either Silver, Black, or Gold. They are sharp, well built, and have the proper holes pre drilled;) I just couldn't afford it on my test dummy:(
Ben

ben62670

Stew wrote: »

Cool project Ben! What tube/tubes are you using?

Ooops I missed you. I am going to be using the 6GM8's. They are a low voltage tube that was used in car radios back in the day. I wanted to stay away from high voltage tubes to keep any noise down in the pre. Cranked all the way this thing is dead dead dead quiet;) The first time I did the test I thought I didn't have thepower one:)
Ben

Systems

ben62670 wrote: »

L/R/G in L/R/G out. This way I can completely bypass all active circuits with a flip of the switch;) I will also need another for the tube buffer. I may just use the buffer on the highs. Right now my XO point is 2000.
Ben

That would be a 3PDT (3 rows of3)scroll down to page 3 part #7301.
http://www.alliedelec.com/Images/Products/Datasheets/BM/C_K_ITT_CANNON/CANDKITTCANNON_INDUSTRIAL-CONTROL_6763000.PDF
however you probably don't need to switch the ground so a DPDT would suffice.

ben62670

I would probably have to go 4pdt if I left the ground connected. What I am trying to do is not even have the SS section getting a signal. If it does there would still be a drain on the signal to ground on the input of the SS section with the selector in passive mode. The volume pot is in between the two SS stages. I could omit the unity stage all together.

jerryj12

stereo_luver wrote: »

Jerry, whats in the secret box?

Chuck

I have seen whats in the box. .. Problem is I dont know what it is..:):D

ben62670

I knew I could trust Jerry:)

ben62670

Well we ran it for PFGA, and it worked very well SS, and the phono. Passive, and tube was not hooked up. Tom had a Jolida tube CD here. One problem is the phono severely needs a rumble filter. The speakers were going **** at an ultra low frequency. I am going to do a 24db rumble filter at 15hz. I have all of these wonderful parts, and no 1uf caps:( I could use .1 but then the two inline signal resistors would be 100k. Thats 200k inline. The thing would most likely be able to play any music at a happy level. I hate not being able to just go and pick up some caps locally. Cost way too much to order just a few pieces. I wish I knew this a few days ago. They would have been here.

ben62670

No rumble filter is needed:) A pair of DC blocking 1uf caps seem to do the trick on the woofer wobble;) The woofer wobble is only when using the phono input;)
Ben

doctorcilantro

Sorry I didn't catch this in the posts....what tubes does the circuit use?

DC

ben62670

doctorcilantro wrote: »

Sorry I didn't catch this in the posts....what tubes does the circuit use?

DC

I am using the 6GM8's. They are low voltage 12v units that used to be used in car stereos in the 50's 60's. I went with this option do to the low voltage required, and the noise associated with them.

I just got in the 6P2T relays for switching between active, passive, and tube stages. I hope these work out well. I am working at a dead silent noise floor which I have with 12db gain in the active stage. The tube stage is a 16db gain, and soon I hope to have a DAC built in that will take advantage of the passive section. I'll report back later.
Ben

analog97

Ben,

You will need 35-45 db gain for most MM cartridges and 60db gain for most low-output MC's. 28db gain will likely not cut enough musturd for you. Maybe I missed something?

Systems

He was refering to the amount of gain in the line stages not the phono section.Which as you mention needs to have a lot more gain.

ben62670

I'm PISSED
All those tubes I bought back in Florida were 6GN8's. I spoke with the guy 3 times about the tubes. I asked for 6GM8's, and told him that I needed those because they were old tubes designed for low voltage. I spoke with the guy three times calling asking for Allen saying this is the guy asking about the 6GM8's. Well moving other projects it has been over a year and a half today I was messing with my experimental tube buffer and wanted to change out the 6GM8 tube because one channels filament was going bad. I pull one out of the box and to hook it up and was looking at the brand and it says 6GN8 just like the rest of them. #**** *$)( &#$)****!!!

mhardy6647

well that sucks. I have to look up the 6GN8 and see what it is...


EDIT: mmmm...
http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_2/6GN8.PDF


triode and a sharp-cutoff pentode...

ben62670

I just spoke to them. They are going to try to help me out. They have 8 of the 6GM8's in stock. That is pretty good so I can at least get mine done. My phone was dying, and I told him it was so they are going to call me tommarrow.

mhardy6647

well, that's cool!

engtaz

Glad to hear their going to do something. Good luck Ben.

engtaz

ben62670

I am pleased that at least I should be able to get myself going. I just like building stuff for friends. I kinda threw out the idea of selling some after the market started to dry up from the other pre's using the same low voltage setup. Even if I can only get those 8 tubes I won't whine too much I will be happy just to have my setup going. The low voltage setup helps keep the noise down and is safer;)

Edit: BTW thanks M for looking that up. I checked it out already, and the only real use that I have seen is high voltage tube can amps;)

ben62670

Well I am so close to being done. The DAC is going in this pre so it will be a tube/passive/active pre.The XO will be separate. I am going to do the case in wood like my custom SDA's. So close on this long journey.
Ben

ben62670

OK some minor design changes. I want to put my DAC in the same case as my tubed pre, tubed phono stage. The case is custom 8 inches high, 14 deep, and 17 wide. I am going to put a switch in there to toggle the power between the tubed phono, and DAC seeing both won't be used at the same time. My concern is noise from the tubed pre affecting my DAC. All transformers will be in a separate section sheilded from the audio gear with a 1/4" wall of aluminum. Any insight would be appreciated.
Thanks
Ben

ben62670

Well after living with a tube pre I have decided to ditch the idea of having it SS/Tube/Passive. The tube pre just does everything so well.

zingo

Fair enough.

ben62670

Here is a Pre I built for a Polkie.

xsmi

Was that a kit? does it have phono? It's gorgeous Ben you do GREAT work!