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PostPosted: 09 Sep 2010, 09:07 
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Location: Christchurch, NZ
Hi guys,

I have yet another project to do. This can be done in a day or 2 !!
Introducing Rod Elliot's DoZ (death of Zen) preamp + power supply board !!

Some specs taken from Rod's site :

Distortion < 0.01%
Output Voltage 10.0 V RMS
Output Impedance 200 Ohms
Minimum Load 3k Ohms
Frequency Response 10Hz - 100kHz (-0.1dB)
Voltage Gain 10dB nominal
Supply Voltage ±15V
Supply Current <10mA


The idea of a discrete opamp + minimalist preamp appealed a lot to me, and it is my 3rd preamp. My other two are based on opamp + mini valve and valve buffer respectively. No fancy components used, just standard 1% metal film resistors where available, and carbon films resistors for values that I cannot find in the shop.

The power supply diagram can also be found on the web, but component values are only available to buyers of his PCB's. I found soldering a little tricky as the leads are very close to each other. Somehow I found this board difficult to solder. I had to continuously wipe the tip for the flux to bond properly.

A 16V AC (700mA) double insulated power pack was chosen to feed into the power supply. This eliminates any mains wiring, and a simple male+female connectors like those used in handphone charges can be used to power the preamp. Enclosure chosen is a black plastic case. Sadly none of the shops sell a small metal case, which I drastically preferred.

To be honest, the circuit boards a little expensive. They are very small and compact as can be seen from the photos. Will begin wiring tomorrow !


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PostPosted: 09 Sep 2010, 12:44 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Nice. I see you saved the enclosure work for last. ;) For those who like PCBs, check out Rod's site. He offers PCBs for many of the more popular projects.

We will be interested in hearing how this preamp mates with the ZCA.
Cheers

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PostPosted: 16 Sep 2010, 00:55 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
Things are running slowly, I screwed up the board and have some traces damaged !!
After trying to fix it, the preamp doesn't work !!
The power supply however, works perfectly.

Anyway I re-ordered another board and some components from Farnell Malaysia LOL.
This is the issue with Malaysian local shops : They workers hardly know much info.
No one knows what a polyester/mica/ceramic cap is. Metal film resistors draw blank looks from them.

Desperation led me to Farnell.

Will provide listening impressions with direct A/B comparisons with :

1. Oatley K272A preamp/headphone amp [Opamp + mini valve)
2. Arjen Helder Tube Buffer+Preamp (12AU7 tube based )

Waiting anxiously for the boards !! :eek:

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PostPosted: 16 Sep 2010, 11:11 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
While waiting for the new boards, I decided to try point to point wiring.
Amazingly I managed to set the bias for 0V with 10mV offset max for each module,
meaning that they work !!

Total cost for these parts : US $5
Rubbish quality trimmers that run when you blow at them !!
Offset from 10mV became 100mV after blowing on them for 5 seconds.

Will hook them up to some RCA connectors and see how they sound tomorrow. :wizard: :geek:


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PostPosted: 17 Sep 2010, 04:57 
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Location: Christchurch, NZ
Initial listening impressions :

- Very very clean sounding, supreme detail retrieval.
- Richness reduced compared to the valve pre..
- Bass is firm and powerful, never gets sloppy.
-Vocals have some magic now. Hard to explain.
-Sonic images more razor sharp in space.

I initially wired the power supply in the box, but it caused some terrible high pitched squeal @ 1000Hz. After relocating the PS away from the preamp modules, hum reduce to nothing.

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PostPosted: 17 Sep 2010, 11:21 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
Current personal ranking :

Winner : Rod's P37
2nd : Arjen Valve preamp+buffer
3rd: Oatley Preamp K272A

Oatley preamp had a very detailed midrange, lacking a bit of texture and bass presence.
The valve preamp had a very rich tone, powerful bass but detail retrieval and imaging not as good as the P37. P37's overall performance beats the other 2 hands down. My huge 2 ways are imaging like a minimonitor now !! :eek: :shock: :up:

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PostPosted: 17 Sep 2010, 22:22 
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Location: Winnipeg, CANADA
:up: Thanks for the notes Dinesh.
What amp were you feeding them into?
Cheers

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PostPosted: 18 Sep 2010, 00:36 
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Hi gio,

I am using the Aussie Amplifier Nxv200 as a reference amp. I feel I am hearing more of Nxv200's personality through Rod's P37A. It just seems to be a clearer window. I am surprised that fatigue has not set in as the overall sound is ultra lively and detailed.

I am not using any volume controls, and the gain is very high. At volume 2/30 on the Mede8er, it gets quite loud. My normal listening sessions are at volume 3.

The preamp is quite sensitive to other electrical equipment turned on in the house. Maybe it's just our "dirty" Malaysian lines but each time someone turns on the shower, there is a slight buzz. A small "click " transient occurs when flourescent lights are turned out outside.

Nevertheless, I can highly recommend this neutral and incredible preamp. :D :thumbsup:
I don't seem to miss my other valve preamp. :mrgreen:
Will I search for another preamp to trump the Elliot P37A ? Maybe not.
My search is over....for now .... :mrgreen:

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PostPosted: 21 Sep 2010, 12:04 
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Awesome. Though I'm sure it won't be long before you get the preamp bug again! :twisted:

You should also try the trio with your ZCA. Would be interesting to see if your order of preference remains the same.

Cheers,
Gio

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PostPosted: 21 Sep 2010, 13:52 
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I have done an evaluation with the ZCA. The results are not too good :bawling:
This P37 preamp brought out some qualities of the ZCA which I never knew existed.

The overall sound was :

1. Slightly bright in midrange
2. Tone was thin and flat.
3. Bass was very good, but inspite of that, it did nothing to add richness to the sound.
4. Dry + Brittle presentation with much weaker dynamics compared to Nxv200
5. Lacking emotional qualities.

This only means that the P37 does not mate well with the ZCA. For that, I have to use the tube buffer to "inject" warmth and tone. Even the Oatley K272A preamp is much, much better with the ZCA.

I think most will find the P37 to be quite aggressive and revealing, with no tone of it's own. For me, that's a good thing as I love a preamp to bring out the character of an amp as best as possible.

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