EZ Tube Mixer Support Thread

[dipfrik]

Problem solved in two stages!

1st: After a chat this mornig with Holger, he advised me to check my grounding.
And he was (as always *g) right. I had a seriuos grounding issue on PSU PCB. After i fixed my star grounding the hiss/buzz was gone as described last night.

2nd: Now i suffered the same buzz problems that Ian, Pierre and Holger had in the past. As mentioned i put an existing 220nF accross the HT-0V entry on the Mixer PCB.

Both problems solved now and it was a big load off my mind, because i expected a total wrong setup/build which comes with big self doubts if i'am ready for the "EZ-Tube-Road".

There is a next problem at the horizon which i need to investigate further, because i believe to hear a low frequency rolloff in "Line" mode in comparison to the "Mic" (with pad activated and gain adjusted) for the same output levels.

For now it's coffee time and finally starting the REDD & Pultec EQ Boards.

Cheers, Sven

 

[dipfrik]

... a few hours later, no cause to make you see REDD if something goes wrong!  ;D

Update: Finished the 1st sound check successfully and the REED EQ is another big surprise! Brilliance baby!
Starting now with the Pultec and be prepared for the grand final listening test of all three EQ's.

 

[dipfrik]

Number 5 is alive and sounds lovely as expected.
Great addition to my rack, especially with the MEQ5 section!

@Ian: Absolutely Top!

Cheers, Sven

 

[dipfrik]

Why one if you can put all three eq's into a channel!
The next days i will stress the channel with different audio material. Be prepared for some recordings.

Night, Sven

 

[dipfrik]

...looks more like the command module from the 1st moon landing

 

[Holger]

12  I/O modules:

 

[nhaudio]

12  I/O modules:

Sexy!

 

[Holger]

Thanks for liking. That's how it looks inside. Four hours of work for one module...

 

[ruffrecords]

Interesting to see you have two switched line inputs per channel Holger. How do you normally use these?

Cheers

Ian

 

[nhaudio]

Interesting to see you have two switched line inputs per channel Holger. How do you normally use these?

Cheers

Ian

Yes Holger, what are the 2 options for line? (transformer or electronicaly debalanced for that extra transistor charm to go with the tubes )

 

[Holger]

nhaudio, no transistors in the signal path.

The two inputs are just for easily switch between (say) tape return/DAW and stage box.

 

[ruffrecords]

Now the 4 into 2 EZTubeMixer has landed safely in Canada I have time to spend on the new 6U channel modules and hopefully also on the Eurorack and maybe even a compressor!!

This evening I ran signal through the 6U unit for the first time and the results are encouraging. First I measured the output level with and without a 620 ohm load and the difference was 1.12dB. My calculator says this means the output impedance at the transformer secondary is a nice low 85 ohms. I am happy with that result.

With the same load I measured 1KHz distortion at 0dBu, +10dBu and +18dbu (as high as my Lindos will read) and got distortion figures of 0.034%, 0.09% and 0.13% respectively. Again I am happy with those figures.

The new layout seems to work OK and the preset pot now being accessible from the front panel is a great improvement.

The only thing that bothers me a little is the frequency response at the top end which seems to vary with the source impedance. If I drive it directly with the Lindos which has an output Z around 50 ohms, I get a +3.7dB peak at 40KHz. The Lindos will not go higher than this but it looks to me like a resonant peak around 50KHz or so. If I then switch in the 20dB pad and up the level by 20dB, the source impedance must now be close to the 150 ohms of the output arm of the pad. In this case, I get a 0.25dB peak at 40KHz. The transformer secondary is loaded with 150K which is the recommended load for the Sowter transformer I am using. I am not quite sure what to do about this. Any ideas would be welcomed.

Cheers

Ian

 

[Holger]

Didn't post for a long time, but still working on my mixer.
I've hit the 1000 working hours mark last week. Wow.
I'm quite confident that I will have eight working channel by mid of October.

 

[chefducuisine]

Holger,

this is stuuuuuuunnning!!! 

 

[ruffrecords]

I think this is going to become a classic. Beautifully crafted, 100% tubes and REDD EQ throughout.

Cheers

Ian

 

[Holger]

A future classic ;-)
Ian, I've decided to use the Helios, they are a bit more versatile than the REDD although the REDDs brilliance filter is a killer.

 

[anjing]

Very  inspiring!
I for one would not mind more photos if time allows! It'a feast for the eyes.

Regards,

Pierre

 

[Holger]

Here we go Pierre

 

[anjing]

Amazing work Holger!

 

[ruffrecords]

One of the most expensive items in the EZTubeMixer design is the input and output transformers so I have been looking for lower cost alternatives. Edcor is a well known for its low cost transformers so I got a couple of their XSM 2.4K/600 transformers. This is physically smaller than the VTB2291 we normally use but it is rated at 2.5 watts which is well over what we need. The primary inductance measures 28H at 100Hz and the secondary is 9.8H at 100Hz, both satisfactory values. I have given this transformer a thorough test and I am pleased to report it performs almost identically to the VTB2291. In both cases, the EZTube output amplifier runs out of steam before the transformers do so I am happy to recommend this as a cheaper alternative. At +26dBu into 600 ohms the distortion with the Edcor was 0,49% ans at +29dBu it became 1.1%. These results are identical to the performance of the VTB2291. Frequency response was -1.8dB at 20Hz but this is entirely due to the 4.7uF output capacitor which is 3dB down at 14Hz with the reflected secondary load. As a separate experiment, I have ordered some 47uF 400V electrolytic capacitors to see if the low end response can be flattened out to 20Hz.

I have not looked at the Edcor mic transformers as they do not come with a screen. Instead, I have started to look at the OEP range of input transformers. I used a couple in the EZTubeMixer 4 into 2 demo mixer, now happily working away in Canada. The ones I tried were the  Z21807E which are a 10K:10K bridging type. I only used them on the input to the headphones monitor amplifier so their performance did not affect the main outs of the mixer, but the phones amp sounded OK to me. I have now purchased two more and I will subject them to more extensive tests and report back.  Initial measurements are encouraging with primary inductance just under 43H. These cost a mere £12 from RS.

Like Edcor, OEP tend to skimp on primary inductance and for a mic transformer this is a key parameter. I was recently made aware of a new OEP mic transformer, the A187A14C which does not currently appear on their web site. I contacted them about it and their technical director sent me a free sample!! I do not have a price for this yet but I believe they plan to make it available through RS. Initial measurements are encouraging. It is a 1+1:8 transformer and the inductance of each primary at 100Hz measures 6.43H so even when wired in parallel for a 1:8 ratio the low end response should be OK.  Secondary inductance measures a healthy 285H - not as good as a Sowter but quite decent.  It is substantially built complete with inter-winding screens and overall screening can. Similar size and shape to the Jensen. Next step is to measure its performance.

I have also looked at some extremely cheap transformers by Triad, also available from RS. They do a 10K:10K transformer that is just £4. I bought a couple  to try out. Although they are specified only for a frequency response from 200Hhz to 15KHz, I thought they might be useful for a VU buffer input transformer. However, initial test have shown they might well be OK for audio work. The model I have is the TY141P. The primary inductance is a little low at 15H but when used as a bridging transformer driven from a low source impedance this is less of an issue. In fact, initial frequency response tests from a low impedance source show a response flat down to 20Hz. There is a 3dB bump at 25KHz but this can probably be tamed by a zobel network on the secondary or small series resistors on the primary. I am now building a test rig so I can test the performance of this and the OEP line bridging transformer at +20dBnm

I'll keep you posted.

Cheers

ian