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PostPosted: 25 May 2013, 14:05 
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Joined: 12 Jun 2009, 15:43
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Location: College Station, Texas
"I generally don't recommend hooking transformers like you indicate. It is done on occasion, but my concerns are related to how identical they really are. If the tolerance is say 20%, then one transformer will try to put power into the windings of the other. Worse if there is some type of phase difference."

One way of overcoming this problem with paralleled secondaries is to give each transformer its own rectifier diodes (which cost very little), and only parallel the connections AFTER the rectifiers. That way, neither one puts anything into the other, even if there is a mismatch. Of course, if the mismatch is large then one transformer may be taking a higher proportion of the burden of powering the amplifier, but at least it won't also be driving into the lower-voltage transformer.

Chris


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PostPosted: 25 May 2013, 21:21 
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Hi, That is what I thought you were going to do. Actually hooking the two transformers in parallel is not a good idea for the reasons mentioned. Rectifying the separate windings and then combining the output is OK. In my dual mono design Poddwatt (not posed yet) I actually have two complete power supplies. Basically two amps on one chassis sharing only the controls. My apologies for misunderstanding what you had in mind.

Good listening
Bruce

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PostPosted: 17 Aug 2013, 10:21 
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Joined: 09 Jun 2013, 10:10
Posts: 6
Location: Stockholm
Hi, Here are greetings from Stockholm with pictures on my finished Mini Blocks. They sound excellent! Image

Image

Image

The construction pictures were easy to follow. I have added a switch on the back panel to disconnect NFB. Comparing with and without NFB I prefer without. Maybe It means some more distortion but I don't hear it. Although the sound is very good with NFB, I think I hear slightly more easy going and less confounded sonics in complex music passages without it. The difference is very small though.

I have also disconnected the cathode bypass capacitors (Elna Silmic II 100uF/6.3V in my case). I did not hear any improvement in sound quality but the loss in gain is very small so I think I leave them disconnected.

I added a ring core choke 2 x 3.3 mH to the PSU (the blue component protruding out from the left side of the PCB), one in series with the 100 ohm resistor and the other in series with the 2 kohm resistor. DC resistance is very small (180 mohm) so there is practically no additional voltage drop. I have not compared the sound with and without them in this circuit, but my experience from all other PSU circuits I have tried them in is that they improve sonics (in theory suppressing high frequency garbage).

I used unshielded silver in teflon from the input RCAs to the 5751s without notable hum. However the Mini Blocks are more sensitive to hum from the sources than my old solid state class A amplifier, but that was a manageable problem that could be solved. Maybe the higher input impedance makes it more sensitive for hum from the sources (100 kohm vs 10 kohm for my solid state amp)?

Thank you Bruce for the very refined and elegant Mini Block design!

Ingvar


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PostPosted: 19 Aug 2013, 08:13 
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Location: USA
I prefer without NFB....IMHO, Quality of sound is more important than modes.....
And I prefer without shunt cathode cap.

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PostPosted: 19 Aug 2013, 15:58 
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Location: Stockholm
I agree, quality of sound is more important. Today I looked at oscilloscope on the square wave output from one of the Mini Blocks when connected to speaker load (one of my main three way speakers with passive filters). Without NFB there were overshot peak of rather large amplitude, however, well damped within a cycle with no ringing. With NFB the overshot peaks were neutralized and very nice square waves were seen (1 kHz, 10 kHz), reflecting the very good design. And guess what, comparing again the sound with and without NFB playing classical music, I now prefer the Mini Blocks with NFB. I think I hear a more balanced and integrated sound stage. I realize though that expectations tend to direct my opinion so I better listen more and for a longer time before any definitive opinion, which I gladly do with these gorgeous Mini Blocks.

Ingvar


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PostPosted: 19 Aug 2013, 21:17 
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Location: Arizona, USA
Hi, I'm glad you like the amplifiers. When using a non-inductive load the overshoot is normally rather well controlled. But the minute you attach speakers all bets are off as they are anything but non-inductive. Crossover components are also non-linear. The NFB was added in a rather small amount (varies a bit, but typically 2.5 db) to handle the "real" world loads that might be attached. From a design perspective it is impossible to guess what someone might attach to it as a load. The NFB compensates for the behavior of many speaker systems to a large degree. It also is designed to take care of the self resonant frequency of the output transformers in the design. All transformers will exhibit some resonant point and the ones specified exhibit it typically at about 70KHZ in this circuit. The NFB restricts the response in that region so that instability is avoided. I use the NFB on in my reference amps. My main speakers are Martin Logan electrostatics and they act like huge capacitors at high frequencies. They also have numerous crossover components. I have not looked at a square wave fed into them with the NFB off, but I would guess it would look rather odd. Very fortunately music is not usually composed of square waves.

For those of you who may be unfamiliar with testing with square waves there are a number of good texts out there on the subject. One thing that is easy to remember is that to reproduce a clean square wave at any particular frequency the actual response of the item tested needs to respond linearly at frequencies roughly 10 times lower and 10 times higher than the test frequency. So in general terms a clean square wave at 1KHZ indicates good linearity down to 100 HZ and up to 10K. Tube amplifiers because of the transformers and other reactive components have problems with square waves over about 5K and below about 250HZ. The Oddwatt family of amplifiers will generally do well with square waves down to around 60-80HZ and up to 7-10K. The evaluation of the waves on a scope is highly subjective and interpretations will often vary with the viewer. I like to use an actual distortion analyzer for distortion measurements and frequently don't list or show the square wave performance. For a comparison SS gear is often direct connected (few or no reactive components in the signal path) and can have clean patterns at both higher and lower frequencies. I have one project SS amp (not published) that has clean ones from about 10Hz to past 50K. If you want to see a graphic view of how something handles square waves set the scope on spectrum analyzer mode. You can then actually see the high frequency performance of the amp by the magnitude and number of peaks.

Good listening
Bruce

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PostPosted: 20 Aug 2013, 13:56 
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Location: Stockholm
Yes, and an interesting property of square waves is that they say something about the amplifier-speaker system behavior. I found a nice interpretation here: http://www.innerfidelity.com/content/he ... nse-page-2 The article is about square wave responses for dynamic headphones but corresponding interpretations of wave forms should apply for conventional speakers as well (please correct me if I'm wrong). The speaker I connected to the Mini Block when I observed square waves yesterday was a Troel Gravesen's TJL3W speaker in transmission line version (see http://www.troelsgravesen.dk/TJL3W.htm). The wave form I saw at 1 kHz with NFB off was similar to the second 300 Hz wave form from the top in the article (p2) except no downward tilt. The interpretation according to the article is "Bright but clean" which corresponds well with my immediate impression although I didn´t chose those words. With NFB on, the uppermost wave form in the article (p2) with only a small or faintly outlined overshot is closest to my observation. It is interpreted according to the article as "Ideal response", so I'm very happy with that observed Mini Blocks - speaker system response.

Ingvar


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PostPosted: 20 Aug 2013, 22:41 
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gofar99 wrote:
One thing that is easy to remember is that to reproduce a clean square wave at any particular frequency the actual response of the item tested needs to respond linearly at frequencies roughly 10 times lower and 10 times higher than the test frequency.
One small correction...

Actually a square wave contains only harmonics above the fundamental. One decade higher is probably what I would consider to be raw minimum (this is 3+ octaves so you get the fundamental plus the first thee harmonics), but it doesn't need any response below the fundamental. A clean square wave at 1kHz may say a lot about the higher frequency response of the system, but it says virtually nothing about the response below 1kHz.

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PostPosted: 21 Aug 2013, 15:24 
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Suncalc wrote:
(this is 3+ octaves so you get the fundamental plus the first thee harmonics)
Oops. Square waves contain odd harmonics so this should have been fundamental plus first four harmonics (i.e. 3, 5, 7, & 9).

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PostPosted: 23 Aug 2013, 20:59 
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Hi Suncalc, You are correct, but when viewed on a scope the top and bottom of a square wave will begin to tilt at a fundamental frequency approximately equal to 10 times the lowest one the item can reproduce well. I don't know the physics behind it, just that it works that way. :soapbox: So when I see someone bragging about a clean 1000 HZ square wave I get a chuckle. Show me the same thing from a tube amp at 50 HZ and then bragging is in order. Clean square waves on the high end are a bit of a mixed bag. For tube gear they should be good at least to 5K and if they are nice at 10K fine. The odd thing is that it is possible to juggle circuit values to get clean ones at higher frequencies, but the consequence is a rather unpleasant sounding amplifier. Most folks would characterize it as "edgy". Also such amps will normally have considerable negative feed back. An undesirable thing in my opinion as it can color the sound as well. A classic case of something that looks good on paper and on the scope but really doesn't sound as great as expected. Solid state stuff like I mentioned earlier can and IMHO should be able to do the 50HZ and 10K+ bit easily.

Good listening
Bruce

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