Tube Phono Equaliser

“Loekie”

Who's gonna challenge Tweety?

Last modified: 20-Apr-2003

Intro, Sept. 2002 ...

A while ago I again had one of those moments: Listening for a full afternoon to old recordings on Vinyl. Some time before I had bought a dozen LP's on a fancy fair and just before we left for the holidays I found the time (and peace of mind) to listen to this black gold. I noticed how well my Tweety phono pre sounded and I knew that I would not like to open Tweety up in order to make the planned modifications, especially since not all changes are in fact improvements and I did not want to take any risk.

In our home we have someone who likes to challenge our Canary Tweety anytime, especially when Tweety is singing. It is our cat called Roderick, a European short haired cat, according to it's passport. And since Roderick is just a bastard cat coming from the local animals' shelter there is no way we will accept arrogant behaviour and therefore we just call him "Loekie". And because we feed him if he behaves well, he does listen to his name and comes when we call him.

Therefore, on this page a description of Loekie, an improved version of Tweety, where I have corrected the most important shortcomings of Tweety. I will use for Loekie (initially) the same power supply as for tweety: Granny. And Loekie will get (sorry for him) the same housing as used for Tweety.

Why Loekie

I came to the conclusion that the RIAA network used in Tweety did not enough possibilities to tweak it to +/- 0.2 dB of the ideal curve (you can read about this on the "RIAA background" page ). No matter how hard I tried, I could not get Tweety to be accurate in terms of RIAA within 0.5dB of the curve. Not that it really matters, as most LP's are not cut within this margin anyway, but it was my wish to build something really good.

Let me at this moment list my (for my family members unbelievable) points for improvement for Tweety.

• I want Loekie to have a RIAA filter with 4 resistors and 2 capacitors
• Tweety does not have a grid resistor on the lower tube of the SRPP, and therefore there is a little chance on HF noise.
• I made use of Elcos to stabilising B+ inside the amplifier housing. I want to change all Elcos for Black gates versions or Audyn caps (smaller values). After that, the amp will not contain regluar type Elcos anymore.
• Therefore, all cathode decoupling caps (now Philips) will be substituted by Black gates.
• The internal wiring of Tweety is based on 0.8mm fine silver with PTFE tubing isolation. I'm considering to use 0.35mm silver wire for Loekie.
• Loekie will be based on a mu-Follower circuit. This will enable me to get some experience which I can use in a later project (Tom and Jerry).

The Schematics

The schematics of Tweety and Loekie to not differ significantly except for the RIAA filter. On the other hand, when implementing the muFollower and all changes as outlined above, it has become another creature after all. Hmm, I'm already concerned about the layout of all these components in the small box that I bought for loekie.

The following list provides more info on the components used:

After the summer holidays I hope to have received all components necessary to build Tom and Jerry and this amp. Since I tried to plan ahead when buying components, I might have bought more than recuired for Tom and Jerry so that I can work on whichever design I like to finish first.

You might wonder why I'm starting yet another phono project. As we deal with hobby hear, common sense is less important that curiosity. And should I, after finishing this project, have one spare I will undoubtfully find someone interested in listening to Tweety or Loekie for a while.

Other Differences

While building Tweety is became clear to me theat whilst a SRPP has a better output impedance it is still not good enough to transmit signals over long pieces of interconnect.

And I had the feeling that connecting the output to the anode of the lower tube of the SRPP makes the impedance even higher but lowered the noise floor (I thought). Hmm, a mu-follower setup will probably have even better S/R ratio and lower output impedance so this will be helpful.

Als I plan to use a couple of real nice audio capacitors for Loekie made by Auricap. For the input section I plan to use a yellow one of 0.47uF and a smaller 0.1uF for the mu-Follower. Auricaps are available for higher voltages, but since B+ in Loekie will remain well below the 300V this does not make sense.

Links

• Tube formulas and spreadsheet; calculate output impedance, miller capacity, gain of tubes
• RIAA cookbook; how to build your RIAA filter even without SPICE tooling. With on the 4th page an analysis of Loekie's filter

© Maarten&Annemarie, 2002-2006