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LTusler

New VTA ST-70

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@rjp ... SHOWOFF!  :D  Just kidding; amazing wiring job ... much cleaner than mine :( 

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Thanks :)  I just copied what I liked from other pictures I saw online. Too bad I'll never get to see it once the bottom's on though :(

 

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5 minutes ago, rjp said:

Too bad I'll never get to see it once the bottom's on though :(

Time for a plexiglass bottom and a stand that sits over a mirror.

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On 5/24/2018 at 9:52 PM, Tarheel TJ said:

 

RJP, I am loving the MiniDSP and ST-70 combo.  Like you, I am still dialing in my settings.  However, compared to my stock LaScalas, it is a monumental improvement!  I am hearing the best hi fi I have ever experienced, including unobtainium setups at high end audio dealerships.

 I too worried about losing fidelity to the AD/DA conversions.  However, what I am hearing is so good I have forgotten all about that.  Maybe you could best this with a really high end all-analog setup, but I would hate to see the price tag.

 

This weekend I had a chance to more thoroughly evaluate the miniDSP 2x4 hd and it definitely introduces some loss of fidelity, but not much.

 

I did this using a line level A/B switch to allow me to listen to the same source either though the miniDSP or direct. I set all filters in the miniDSP off. My goal was to hear only the effects of the miniDSP's signal path when it is not trying to change the sound. I figure this is a baseline of the best it can do as far as preserving fidelity.

 

I could hear a slight loss of high end clarity and spaciousness through the miniDSP path compared to the direct path. My conclusion from this is the the miniDSP's A/D, D/A, and signal path circuitry introduce a slight loss in fidelity. Of course this must be the case, but I wasn't sure if it would be audible. It is.

 

The loss of fidelity is slight, and I could easily see how the benefits of equalization it brings could outweigh this. So pretty much as expected, it is a trade-off as to which is more important in the particular situation and the particular listener.

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RJP, I never tried running a straight signal through the MiniDSP like you did.  In my case, I replaced the stock AL-3 crossover and went straight to a multi-amp setup.  The improvement in fidelity was huge.  Whatever loss in fidelity the MiniDSP introduces, and I am sure there is a little, it was more than offset by removing the passive crossover entirely.  I am loving what I am hearing right now!

 

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Know this is an older thread, but did not want to start a "new" one.

 

Any of you ST-70 builders ... are you getting enough output from this amp??  Running mine with a pair of Forte's and have to turn it way, way up (3 clicks) from max to get 85-90dB from my speakers :(  (Compared to maybe 20% on the volume knob of my same wattage Marantz 1060)  No clue :( 

 

Any thoughts are appreciated.

 

Cheers, Emile

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2 minutes ago, Emile said:

Know this is an older thread, but did not want to start a "new" one.

 

Any of you ST-70 builders ... are you getting enough output from this amp??  Running mine with a pair of Forte's and have to turn it way, way up (3 clicks) from max to get 85-90dB from my speakers :(  (Compared to maybe 20% on the volume knob of my same wattage Marantz 1060)  No clue :( 

 

Any thoughts are appreciated.

 

Cheers, Emile

I got an original that had been refreshed and it has no power problem. So im thinking there is something not right with yours.

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2 minutes ago, ricktate said:

I got an original that had been refreshed and it has no power problem. So im thinking there is something not right with yours.

Rick, Thanks :D  Will try to "dig into" it. Hopefully will get some more advise :( 

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Guest wdecho

It may have to do with gain. Try a preamplifier with gain first before trying something else unless others that have a ST-70 say different. I thought that the ST-70 does best with preamplifier that has gain. Many of the older tube amplifiers require a preamplifier and even some of the newer ones. Nothing to do with power or quality of sound or how far you have to turn volume knob. If you are happy with the volume of sound as is do not fret over it. Many are happy with just a passive potentiometer used on a power amplifier to control volume. 

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7 minutes ago, wdecho said:

It may have to do with gain. Try a preamplifier with gain first before trying something else

@wdecho ... Thanks!  Switched from a Dynaco PAT-4 to a JVC-3030 preamp.  (Supposedly pretty good, but don't think it has any additional gain.)  Sound is much better than the PAT-4 :)  And ... sound from the ST-70 is excellent.  

 

But ... maybe I'm just using it wrong :(  ... have the optional attenuator installed on it ... maybe this does NOT work like a regular volume pot?  From Bob Latino's VTA site ...

Quote

A 21 step, stepped attenuator kit to install in place of the unused stereo/mono switch to allow you to use your ST-70 amp without a preamp !
This is a great option if you only have one signal source for your ST-70 like a CD player. The attenuator may also be used WITH A PREAMP.
If you turn the control all the way to the right there is complete pass through of the signal with no attenuation or alteration of the signal.
You may also use the attenuator as a master level control to effectively reduce the gain of the amp.

Unquote

 

OK; will try it with the amp at 100% and "adjust" it with the pre-amp.

 

Many thanks! :D 

 

Cheers, Emile

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58 minutes ago, Emile said:

OK; will try it with the amp at 100% and "adjust" it with the pre-amp.

That's the way to do it.  I have the same amp and use it with an Emotiva PT-100 preamp, I get plenty of volume.

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Thanks @jvs1670 and @wdecho ... yes; plenty of volume, but was trying "louder is better" with some Pro KP-3002's.  When I use the ST-70, I have to go to 100/100% volume to get to 88dB's  (yes; louder than my normal listening volume) ... but when I use a 35W vintage amp I get to 88dB at about 20% volume level ... and much more when I "turn it up."  Just puzzled ... Thanks :D 

Cheers, Emile

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Got a response from Bob Latino ... seems like the "attenuator" is slightly different from a "regular" volume control ...

 

Quote

The stepped attenuator is set between the input jacks and the driver board IN eyelets. Normally it blocks some of the voltage going to the driver board inputs. If you turn the attenuator all the way to the right, it is basically "out of the circuit". What goes into the input jacks goes directly to the input eyelet on the driver board. This is the best way to use the attenuator with a preamp.

 

The attenuator does not give much increase in volume during the first 6 or 8 steps. At about 11 to 1 o'clock the volume will get louder. Using my own ST-70 with the attenuator, I find that about 2 to 3 o'clock the music gets very loud with the CD player I use. CD players all give out about 2 volts. The ST-70 only needs about 1.2 to 1.3 volts IN for full output of the amp.

 

If you have a preamp, do what I mentioned above >  If you turn the attenuator all the way to the right, it is basically "out of the circuit". Don't use BOTH the preamp and the attenuator to control volume if you have a preamp - just use the preamp to control volume and leave the attenuator turned all the way to the right.

Unquote

 

But still not sure why I don't get more than 88dB's :( 

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Guest wdecho

Your source does not have enough voltage to drive the amplifier to full output. When using one of my buffer preamps, no gain, with a Firstwatt amplifier using a CD player I have enough volume for my needs. In fact more than enough. But when using my vinyl turntable with built in preamp it does not have enough voltage out and a preamplifier with gain is required. Where the volume control needle lies has nothing to do with gain as such. That depends on the kind of potentiometer used. With a linear pot slightly open will be too much volume but with a audio pot it may take the 3:00 position to attain the same level in sound as with the linear pot. If your source provides 1.4 V out the amplifier will attain full output but where your volume pointer lies will depend on the type of potentiometer used. I often change the kind of potentiometer in my preamplifier and amplifiers I build to reach the right position I like. It is a waste of a potentiometer if you do not need at least 2:00 to 3:00 a clock to attain full output. You will not have as much control of the volume if you reach too loud at say 9:00 a clock. I hope this makes it more clear. I hear it stated all the time, "boy my amplifier is so powerful, it reaches too loud at 8:00." In reality in means nothing. Manufactures use this tactic to take advantage of this misconception all the time where the better usually more expensive amps, preamps design their products where one has full use of the potentiometer.  

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18 hours ago, Emile said:

Know this is an older thread, but did not want to start a "new" one.

 

Any of you ST-70 builders ... are you getting enough output from this amp??  Running mine with a pair of Forte's and have to turn it way, way up (3 clicks) from max to get 85-90dB from my speakers :(  (Compared to maybe 20% on the volume knob of my same wattage Marantz 1060)  No clue :( 

 

Any thoughts are appreciated.

 

Cheers, Emile

Emile,

I think I know what you are talking about, but after reading some of the interveaning replies I am a little unsure now, so I will just speak about my experience with stepped attenuators.

 

 I had a "regular" volume pot on my VTA70 before I installed a 21 step attenuator. My first reaction was, "Why is there so much less volume now?"

 

There is not less volume. It has just been redistributed.

 

The difference was entirely due to the taper of the designs.

 

The stepped attenuator (at least the most common ones) have steps that are roughly 3dB each. This means that just 2 steps down from full volume is half power. This is known as a logarithmic taper (a.k.a. Audio taper). The potentiometer I was using was a linear taper. So even though the two devices spanned a total resistance of 100K Ohms, they did so in very different ways. Most volume controls on consumer stuff are linear taper, and consequently that's what a lot of us are used to. Most audiophile stuff, however, tends to be logorithmic. WIth a linear taper control the volume initially increases quickly as you turn the knob clockwise, and then slows down. I prefer linear personally.

 

You can get a stepped attenuator in linear as well if you like the "advantage" of discrete steps rather than continuous adjustment. Same price on ebay. Drop in replacement.

 

 

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Emile,

Are you running an original ST70 or a modified version? Where is the attenuator you mention installed? 

 

BOTH a stepped attenuator and ANY volume pot of ANY taper will be effectively out of the circuit at full clockwise rotation (well, unless it's wired up wrong).

 

What is you input source?

 

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12 hours ago, wdecho said:

the volume control needle lies has nothing to do with gain as such. That depends on the kind of potentiometer used. With a linear pot slightly open will be too much volume but with a audio pot it may take the 3:00 position to attain the same level in sound as with the linear pot

 

1 hour ago, rjp said:

The stepped attenuator (at least the most common ones) have steps that are roughly 3dB each. This means that just 2 steps down from full volume is half power. This is known as a logarithmic taper (a.k.a. Audio taper)

 

Awesome guys ... thanks a lot!  Was ready to sell this ST-70 ... Haha; will keep it for a while :) 

 

Regards, Emile

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2 hours ago, rjp said:

Are you running an original ST70 or a modified version? Where is the attenuator you mention installed?

rjp, thanks! I have the (standard) Bob Latino ST-70, with the 21 step attenuator (and Russian 6550 tubes and caps). 

 

Understand the linear vs log attenuation now - thanks!  But ... still a bit puzzled why the 35 Watt ST-70 gives me a lot less volume than my 35 Watt vintage Kenwood. Well, moot point now as my wife is returning from a trip ... meaning won't be checking max sound levels till she leaves again :D 

 

Cheers, Emile

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Guest wdecho
3 hours ago, Emile said:

rjp, thanks! I have the (standard) Bob Latino ST-70, with the 21 step attenuator (and Russian 6550 tubes and caps). 

 

Understand the linear vs log attenuation now - thanks!  But ... still a bit puzzled why the 35 Watt ST-70 gives me a lot less volume than my 35 Watt vintage Kenwood. Well, moot point now as my wife is returning from a trip ... meaning won't be checking max sound levels till she leaves again :D 

 

Cheers, Emile

I doubt it unless one of the amps are fudging. The only way to know for sure is to measure output watts at clipping but that will require an oscilloscope, signal generator and multimeter to know exactly how much power each one has at clipping. Moot point with our speakers though , because either one should have more than enough power for 95db or higher speakers in most any room. 

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