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Old 23rd February 2012, 09:07 PM   #311
RollE2k is offline RollE2k  Sweden
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Two questions!
If using Buffalo II/III with 1:1 transformer like Lundahl LL1690, then the output voltage will be approx 1.17vRMS? and if using 2:1 slightly below 0.6vRMS? or have i understand everything wrong?
Also, if running 1:1 how do calculate output impedance? Why i ask is because it will be feeding my single ended line stage, which does have approx 16k input impedance. So lower output impedance would be better for me!

Last edited by RollE2k; 23rd February 2012 at 09:16 PM.
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Old 24th February 2012, 12:53 AM   #312
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Depends on how many DACs are run in parallel. For the most part it will be four or eight.

We are talking using 1:1 in voltage mode?

With four the output should be close to 2V, but add 2 x 330R will be about 1.3V (we are talking RMS voltages). With those resistors added (both grounded), the output impedance will be lower. Without them about 500 Ohm (takes into account some DCR in the transformer).

16K should not be a problem.

Cheers, Joe R.
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Last edited by Joe Rasmussen; 24th February 2012 at 12:58 AM.
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Old 24th February 2012, 05:51 AM   #313
RollE2k is offline RollE2k  Sweden
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Thanks alot Joe.
But how do i calculate the output impedance (if i want to check with some other transformers) especially when looking at a 2:1. Also, how can the output voltage be close to 2v? isn't peak to peak voltage (3.3v out from dac) just converted to RMS with a 0.3535 ratio?
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Old 24th February 2012, 08:30 AM   #314
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Joe,

Whatís your opinion of the use of a differential high gain JFET - tube cascode with (really) low ohmic IV resistors for IV conversion (like in one of your/Allenís MC preamp designs) to start with? That stage could be followed by a balanced to unbalanced cathode follower like in Broskyís Unbalancer (see picture).
I do understand that my question is a bit off topic because no transformers are needed then, but since you have explored high gain Ė low noise amplifiers in the past so much I wonder why you didnít choose that as your point of departure.
I do understand however that many roads lead to Rome.

Peter

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Old 24th February 2012, 08:36 AM   #315
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The best way is simply to measure it.

Example: Use a 1K resistor as reference. Play a sine wave, preferably at 0dBFS (full scale) and measure the output voltage. Then put 1K resistor across the output and measure the voltage it has dropped down to.

The rest is simple Ohm's Law:

Say you got 2V first and then 1V after with 1K load. The output impedance is the same as the 1K resistor. If it drops less than that, say 1.6V, then we can calculate:

2/1.6 = 1.25

1.25 * 1000 = 1250

1250 - 1000 = 250 Ohm

That's it.

I suspect with four DAC's in parallel you will get near 500 Ohm.

With 330R (2) to ground, it should be about 300 Ohm.

Both will drive 16K. In fact, it will drive 5K (less than 1dB insertion loss).

Cheers, Joe R.
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Old 24th February 2012, 08:42 AM   #316
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Quote:
Originally Posted by pietjers View Post
Joe,

Whatís your opinion of the use of a differential high gain JFET - tube cascode with (really) low ohmic IV resistors for IV conversion (like in one of your/Allenís MC preamp designs) to start with? That stage could be followed by a balanced to unbalanced cathode follower like in Broskyís Unbalancer (see picture)...

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Hi Peter

While not the exact arrangement as you state, but yes, something very similar we have done on a regular basis using a device that is basically a "diamond transistor", very very low noise, huge bandwidth and as much gain as you need. It does have asymmetrical diff inputs, but with 1-3R that makes no difference.

If you have seen the Vacuum State Sony XA5400ES and Yamaha CD-S1000 and CD-S2000 that we have done, that's what they are doing.

Many ways to apply this one.

Cheers, Joe R.
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Old 25th February 2012, 08:36 PM   #317
GaryB is offline GaryB  United States
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The transformers I used are Tamura GA80071 as used in the Yamaha PM1000 mixing console. They are 600:1350 ohms which translates to a 1:1.5 step up ratio . . . I followed them with a diamond buffer to make sure there was sufficient drive. I loaded the secondary of the transformers with 15k ohms. I suspect that tweaking the loading of the primary and/or the secondary will have benefits . . .
After listening for the last week or so, I've gotten a feel the sound with this transformer + buffer output stage. It is nicely transparent but just a touch on the bright side. I suspect that the 15k load on the transformer secondary is a touch too high.

To understand what was going on, I put one of the transformers on the scope and tested it with square waves from 100hz to >100khz. The transformer response falls off rapidly between 100khz and 220khz but was quite flat up to a bit over 100khz. The leading edge of the square wave had a bit of ringing on it which could be damped completely by loading the secondary with 3600ohms. So I listened to the system with that load, but it was obvious that 3600ohms was over damping the system and squashing the dynamics. Going back to the scope, I saw that playing with higher loading didn't make an obvious difference in the square wave response. So it was back to listening tests to adjust the load to get something that sounded right in my system.
It turned out that a 10k load was just right. It still had all of the dynamics that were lost with a 3.6k load but it took the edge of the highs that I'd noted originally with a 15k load.

This mirrors my experience using transformer for MC phono cartridges. Playing with loading is critical to getting the sound dialed in. I'd recommend that anyone playing with transformers experiment a bit with the loading.

---Gary
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Old 26th February 2012, 12:58 PM   #318
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To understand what was going on, I put one of the transformers on the scope and tested it with square waves from 100hz to >100khz.
Digital (through the dac) or analog?
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Old 26th February 2012, 02:23 PM   #319
GaryB is offline GaryB  United States
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Digital (through the dac) or analog?
Brian,
It was analog outside of the dac. I used a surplus Tek square wave generator and Fluke scopemeter. I added resistance in series with the square wave generator to mimic the output impedance of the Buffalo.
---Gary
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Old 1st March 2012, 03:32 PM   #320
chaos is offline chaos  Austria
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Default buffalo III + mc-stepup...

hi! just ordered a buffalo III with ivy. at first i plan to listen to this combo to get something like a reference.
later:
dac output loaded with 2x 1R (the way mr. rasmussen suggests, should give around 10mv on the primary), S&B tx103 1:5, so we get 50mv. a mu-follower with 5842(~120v over each valve, 10ma, cathode resistor 150R)and a bartolucci "layered plate choke", 80H, between the 5842īs.
is something wrong with this plan or is it worth trying?
(have nearly all the parts lying around, including superreg from allen wright and choke input psu..)


thx, klaus

Last edited by chaos; 1st March 2012 at 03:41 PM.
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