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Pre Amp Compatibility with Hypex UCD700HG Mono Blocks....

Weeman1973

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I'm ready to pull the trigger on a pair of Hypex UCD700HG Mono Blocks but only have a Tisbury Passive Pre Amp currently.

I want to reduce box count / cables everywhere so also looking at a Mk I Auralic Altair Streaming DAC to use as the main source and pre amp.

As I'm not all that clued up on input sensitivity etc. when matching pre and power combo's, I was wondering if someone can advise if this combo would be OK together or whether or not I may encounter some issues?

I would also swap out the Auralic for the Tisbury passive when using my TT and using that but would need to adapt some cabling as the Hypex Power Amps only have XLR inputs. Again, would there be any issues there?

Thanks.  

 

Lurch

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Rca to xlr converters or a cable made up this way would sort that out. As for compatibility I will leave that for others to answer as I use an MFA Baby Reference passive with my Nords. 

 

Mr.Ian

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Research the output impedance of the pre amp, i think it reaches 2.5kohm and the input impedance of the power amp buffers 30kohm?. 

An input impedance of 10x the output is generally thought to be the absolute minimum ratio, so you are border line, 20x would be better.  

You might get some roll off at the frequency extremes. 

Having said that a passive with a class d might be too squeeky clean sharp at the top end, i always found a valve pre worked well with hypex but some treble roll off might help. 

 

Lurch

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My MFA passive is a stunning combo with the Nords I got of you Ian. No idea of the output impedence of the Baby Reference but it works. 

 
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zeta4

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The o/p impedance of a TVC like the MFA will depend on the o/p impedance of what's driving it. The action of a TVC means that at most volume settings the reflected o/p impedance will be lower. For example if the o/p impedance of a phono stage is say 1K and the TVC is set for say -12db ( 4 : 1 step down ) then the o/p impedance that the power amp sees is 1000/ 4x4 or 62,5 ohms .  This is different from a resistive based passive pre.

 

newlash09

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My MFA passive is a stunning combo with the Nords I got of you Ian. No idea of the output impedence of the Baby Reference but it works. 
+1 :)

Mine is no where close to the above league. But I've sent off my tube amps for a much needed service. And iam presently running my Pmc twenty 26, through a MFA classic passive preamp, driving a pair of inexpensive Nuprime STA-9 class -D mono blocks in bridged mode. And the sound is really stunning. This is the cheapest amplification I've tried with the pmc's,  but surprisingly this combo got the best out of them so far. 

Edit : I've tried different implementations of power amps. And some how the MFA passive preamps,  seem to work well with all kinds of power amps.

 
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Mr.Ian

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Guess its all about synergy, it could be that its my speakers, impulse h2, that need the valves. I know the top end was too bright when i used ss pre and chord power amps with them. 

 

uzzy

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It is a good question applying to not just Hypex but most power amps.  For the life of me I cannot understand why an amp made today would want to see a signal of less than 1.5 volts (and ideally 2) as this was the output of CD players from the day they hit the market (which also amazed me as all the preamps of that period many years ago had input sensitivity of 125mv). 

My problem in recent years is matching my SP9 to power amps that have an input sensitivity of less than 1 volt.  

Anyway back to your question - I went to the Hypex site for FAQs and yes there was one about input sensitivity .. perhaps @tuga or some other boffin in here can translate the answer from Hypex  


15 Q: How do I calculate the input sensitivity for my application?


A: Input sensitivity is dependent on the desired output power, the impedance of the speaker and the gain of the amplifier. Naturally the desired output power cannot be higher than the rated maximum of the amplifier. In the example the input sensitivity for a 400W output in 4 Ohm is calculated. The gain and maximum output power is given in the datasheet. The speaker load is dependent on the user application.

inputsensitivity.png


 

Lawrence001

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The o/p impedance of a TVC like the MFA will depend on the o/p impedance of what's driving it. The action of a TVC means that at most volume settings the reflected o/p impedance will be lower. For example if the o/p impedance of a phono stage is say 1K and the TVC is set for say -12db ( 4 : 1 step down ) then the o/p impedance that the power amp sees is 1000/ 4x4 or 62,5 ohms .  This is different from a resistive based passive pre.
I didn't know this. If true it's a very strong argument for TVCs and maybe autoformers etc.

Can you add a link to a paper or article on how input and output impedance behave in a transformer please?
 

rabski

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I didn't know this. If true it's a very strong argument for TVCs and maybe autoformers etc.

Can you add a link to a paper or article on how input and output impedance behave in a transformer please?
It most certainly is true and it's quite simply how any and every transformer works. A transformer has a ratio, and converts not only AC voltage but also impedance. The calculation is the impedance A reflected on the primary winding of a transformer is the impedance B divided by C squared, where C is the turns ratio of the transformer and B is the impedance loaded on the secondary winding.

This is the key factor with moving coil step-up transformers and although it's straightforward, it seemingly often gets ignored or not worked out correctly. For example, a normal MM phono stage has an input impedance of 47K ohms. If you use a 1:20 step-up, then the impedance on the primary winding (so what the cartridge 'sees') is 47000 divided by 20 x 20, therefore 117.5 ohms. With a 1:10 step-up the cartridge sees 470 ohms, and although 1:10 may appear more suitable in terms of gain, if the cartridge works ideally with around 100 ohms loading, 1:20 will be better. Every system has a volume control, so a source that is quite high output isn't a major issue.

It's only more complicated with a TVC because for every volume setting, the ratio will be different. However, as Geoff points out, the ratio is 1:1 at maximum volume, so effectively the output imedance of the TVC is the same as the output impedance of the source. At any lower voume setting, the output impedance will be lower than that of the source, which will never be a problem.

The 'golden rule' is 1:10, in terms of source output impedance and power amp input impedance. I've seen some people suggest that the higher the ratio the better, but there is no actual point. In fact, you can probably get away with as low as 1:5 before there is any audible effect in most setups, but 1:10 is sensible and avoids any potential issues with frequency extremes or attenuation.

With an active preamp, the output impedance of the preamp needs to be taken into account. With a TVC, the output impedance of the source needs to be taken into account.

 
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hifinutt

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Guess its all about synergy, it could be that its my speakers, impulse h2, that need the valves. I know the top end was too bright when i used ss pre and chord power amps with them. 
yes that combo would be great with my tannoys but MOST certainly not with impulse !!

 

Lawrence001

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It most certainly is true and it's quite simply how any and every transformer works. A transformer has a ratio, and converts not only AC voltage but also impedance. The calculation is the impedance A reflected on the primary winding of a transformer is the impedance B divided by C squared, where C is the turns ratio of the transformer and B is the impedance loaded on the secondary winding.
This is the key factor with moving coil step-up transformers and although it's straightforward, it seemingly often gets ignored or not worked out correctly. For example, a normal MM phono stage has an input impedance of 47K ohms. If you use a 1:20 step-up, then the impedance on the primary winding (so what the cartridge 'sees') is 47000 divided by 20 x 20, therefore 117.5 ohms. With a 1:10 step-up the cartridge sees 470 ohms, and although 1:10 may appear more suitable in terms of gain, if the cartridge works ideally with around 100 ohms loading, 1:20 will be better. Every system has a volume control, so a source that is quite high output isn't a major issue.
It's only more complicated with a TVC because for every volume setting, the ratio will be different. However, as Geoff points out, the ratio is 1:1 at maximum volume, so effectively the output imedance of the TVC is the same as the output impedance of the source. At any lower voume setting, the output impedance will be lower than that of the source, which will never be a problem.
The 'golden rule' is 1:10, in terms of source output impedance and power amp input impedance. I've seen some people suggest that the higher the ratio the better, but there is no actual point. In fact, you can probably get away with as low as 1:5 before there is any audible effect in most setups, but 1:10 is sensible and avoids any potential issues with frequency extremes or attenuation.
With an active preamp, the output impedance of the preamp needs to be taken into account. With a TVC, the output impedance of the source needs to be taken into account.
I wish I knew this before I sold my Django pre. I might have kept it. Although I later sold my Ice Power amp anyway so it's academic now.
 

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