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New DIY Phono Pre

dnewma04

dnewma04

The Healer
I had wanted to post some pics of the progress as it was occurring but decided to wait for the fest for it's public unveiling. This was the result of quite an extensive search for the right design and once we found the pre we wanted to build, there was a fair amount of time spent planning the preamp's execution. Squidward and I came up with the same idea on the same day. We both essentially "found" the Octal version of the Cornet literally on the same morning. Reviews seemed excellent all around and me being the loctal fan that I am noticed immediately that both octals used had an equivalent loctal based tube. Now we had our tube selection, we had our schematic and we had a general idea of the layout we wanted. It was time to start drawing up the proposed chassis. I drew it up on my end and posted what I thought would be a good layout on photobucket then asked Ed what he was thinking in terms of layout. He went on to describe what I had drawn almost exactly. There were some modest changes like the position of the RCAs but for the most part, it was dead on.

Squidward hates chassis work and ordering parts, I'm stupid and don't know any better so I handled that part. Ed was in charge of the component layout and the solder slinging.

Here are some pics of various stages of development.

initial rough layout when considering using damper diodes:

layout.jpg


roughed out layout when deciding on a full wave rectifier:

layout-1.jpg


With the changes to the RCA jack position:

layout-2.jpg




Let the chassis begin...Since this was to be done on a tight budget, exotic woods were out of the question, so by default I used poplar for workability and more importantly the dirt cheapness.

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.090 aluminum top plate drilled, punch primed with the wood frame surrounding it.
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Top plate painted and pieces just resting in their final position:

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With all of the components mounted securely:

DSC00732.jpg


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The subsequent handoff to squid resulted in the following wiring progress.

rough layout of the front end:

P2220004.JPG


DC heat PSU module, Ed tried to design it to be a fairly modular design.

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PSU wired up: DC heat module connected to the same mounting holes as the power transformer.

P4250005.JPG


Higher res pics can be found here:

http://tubehacker.com/events/loctal_riaa/
 
Cool stuff! :thmbsp: I'm a loktal fan as well, I think a williamson project I'm working on is gonna use them.

Got a schematic of the RIAA pre?
 
I was pretty happy with the sound on Friday night, even with only a few minutes of break in time, but I went in with fairly low expectations of performance. For one thing, it's virtually silent with no signal present. There is a volume dependent hum but even at max volume, it was pretty quiet on Klipsch Cornwalls which are obviously very sensitive speakers. On Saturday and Sunday, I think there was an overall improvement in the sound. With only 36-38dB of gain, it could stand to be a little hotter but with Ed's MM cart, it was more than enough to drive his 5w SE amp to comfortably loud levels.

We have yet to really play around with any kind of tube rolling and it doesn't really contain a single expensive cap in it. We have some revisions we want to make for the next one we have planned. Some minor layout changes, I would like to add LOMC capability, and we have some ideas for better shielding in the works as well.

I'm not the best at describing what I hear, so I can say that it sounded very good to my ears and didn't give up an awful lot outside of some gain to an Art Audio Vinyl One that got some play in the room. It was better, no doubt, but for a first prototype of sorts, it didn't get embarrassed by any stretch.

I hope that some others that did get an opportunity to listen who might be more objective will pop in with their own comments.
 
Bricktop, if you google hagerman octal cornet, you'll find the schematic we used. I can work up a schematic for the the PSU that we used and post it later on. Thanks for the comments, I know I'm proud of the result and positive that Ed is, or should be.
 
Hee hee... we'll have to figure out some way to get Dave's preamp playing in the same system as my loktal-based Magnavox AMP175/8802 clone project. Using 7C5 (loktal 6V6) as output in that amp.

Got all the holes drilled/punched on that... now time for bodywork and paint. It's a rack-mount chassis; think I'll paint the front panel satin black, and the main part of the chassis gold... sounds like an appropriate scheme, being it's basically a Magnavox... :D

Regards,
Gordon.
 
Hagtech PCB $119 Shipped Anywhere.

I thought I would mention that you can purchase the PCB from Jim Hagerman himself with directions at www.hagtech.com. It would be under Cornet II and you can search on his site for the octal tweak to his PCB. Jim is a great guy to do business with and according to his blog is going through some trying times personally. I have a tweaked-out C2 that I built with Jim's PCB and it is one of my favorite pieces of gear. Peace. Robert :music:
 
hey-Hey!!!,
Nice job there you two!

I have a small UTC Ouncer TX that might do the job. O-16 and I think it is a 12:1 ratio. Getting a coil in place of a grid resistor is usually a nice side benefit. They come up on ebay a whole lot cheaper than the 4722 and 15095 Peerless units. The input TX is a very sensitive spot; what works for one cartridge will make another sund like crap.

I've also seen an OP amp used to good effect as the input stage for a MC cartridge. As with anything audio, the right one is probably not going to be the first one you pick out...:)
cheers,
Douglas
 
One of my side projects is a JFET passive RIAA amp, I can't really justify putting a whole lot of money into a tubed RIAA pre for how much i listen to records, so this i think is a good compromise...
 
schematic ...

Hello,
thanks for posting the pictures, very good mechanical work, congratulations.:thmbsp:

A few thoughts about the schematic.
I downloaded the schematic pdf from
http://www.hagtech.com/cornet.html.
The input capacitance is approximately C Miller multiplied by the gain of the input stage.
In this circuit the output resistance of the first stage and C7 makes the first RIAA low pass
filter. Thus the gain of this stage depends in the signal frequency and the input capacitance
does the same. This problem is increased by using octal triodes.

In e.g. solid state, pentode, and cascode amp stages the transconductance
(mutal conductance, gm, s, etc.) characterises the amplifier.
Thus why do you do the same with a triode? Why do you build a triode amp "transistor like"?:scratch2:

Kind regards,
Darius :)
 

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oldeurope said:
Hello,
thanks for posting the pictures, very good mechanical work, congratulations.:thmbsp:

A few thoughts about the schematic.
I downloaded the schematic pdf from
http://www.hagtech.com/cornet.html.
The input capacitance is approximately C Miller multiplied by the gain of the input stage.
In this circuit the output resistance of the first stage and C7 makes the first RIAA low pass
filter. Thus the gain of this stage depends in the signal frequency and the input capacitance
does the same. This problem is increased by using octal triodes.

In e.g. solid state, pentode, and cascode amp stages the transconductance
(mutal conductance, gm, s, etc.) characterises the amplifier.
Thus why do you do the same with a triode? Why do you build a triode amp "transistor like"?:scratch2:

Kind regards,
Darius :)
Click to expand...

Hi Darius,

I have no idea. We found the Hagerman schematic, read great things about it's performance and the tube types allowed us swap for loctal equivalents to save money.
 
oldeurope said:
Hello,
thanks for posting the pictures, very good mechanical work, congratulations.:thmbsp:

A few thoughts about the schematic.
I downloaded the schematic pdf from
http://www.hagtech.com/cornet.html.
The input capacitance is approximately C Miller multiplied by the gain of the input stage.
In this circuit the output resistance of the first stage and C7 makes the first RIAA low pass
filter. Thus the gain of this stage depends in the signal frequency and the input capacitance
does the same. This problem is increased by using octal triodes.
Click to expand...

According to the actual octal cornet schematic that he used at http://www.hagtech.com/pdf/cornetoctal.pdf, the first stage is pretty well isolated by a 47k resistor, making gain fairly constant over the audio band, then input capacitance is constant as well.

oldeurope said:
In e.g. solid state, pentode, and cascode amp stages the transconductance
(mutal conductance, gm, s, etc.) characterises the amplifier.
Thus why do you do the same with a triode? Why do you build a triode amp "transistor like"?:scratch2:
Click to expand...

Email Hagerman.
 
First off, we are humbled by the many congratulations. Especially from the likes of master craftsmen like Pak, Darius, and Bricktop. It's the DIY community here at AK and other places that gave us the ideas and encouragement to take on such a project in the first place.

The signal path circuitry, as Dave mentioned, is almost a direct copy of the "Octal Cornet" by Jim Hagerman. The "octal" schematic differs from the original Cornet slightly, and does not contain enhancements (CCS, perhaps?) found in the Cornet II. We chose this circuit for our signal path because it seemed well received, and with some minor modification would work with Loctal tubes, which we wanted to build around. 6SN7 is expensive, 7N7 is not.

The power supply is still a work in progress. We overestimated the load of the preamp, and we did not draw critical current through the chokes in the power supply. We have a little work to do to correct this, but part of the exercise was to teach me and Dave more about power supplies, and we are getting quite the education.

It is possible to buy PCBs direct from Hagerman, and avoid a lot of trial and error and general fabrication headache. If anyone is getting in to DIY, I'd strongly encourage them to try a PCb as a first project. My first project was a DIYTube ST-35, and it was a great way to get introduced to this hobby. PCBs streamline construction, and help remove sources of builder error. For this project tho, we wanted a level of flexibility that we did not feel that a PCB afforded us, and we were willing to take some lumps along the way. Hat's off to Hagerman for providing a schematic, and a superior product. We in no way intend to sell anyone else's intelectual property, use of this schematic will be for personal use only (and maybe one for a friend). I would also like to note that the version we chose to build is no longer available as a PCB (to my knowledge), and when it was you still had to do some level of P2P work to get it to work with loctal/octal tubes (including cutting up the PCB, again, to my limited knowledge).

Dave's execution of the chassis went well beyond my expectations for craftsmanship. I was extremely impressed. People who saw the preamp at the fest often mistook it for either a headphone or single ended amp, to give you an idea of the physical presence of the thing.
 
Darius, I am not technically competent to answer your questions. Thank you for giving me another aspect of this project to ponder, tho.

There's something else you might find amusing: Your direct coupled phono preamp schematic was the original one we had thought to try. We decided on the Cornet because it seemed like less of an ambitious build for a first P2P phono preamp project, tho. Maybe there will be a DaRIAAus (our name for your schematic) at next year's Fest! (And maybe you'd like to come and see it?)
 
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