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Old 3rd February 2011, 02:29 PM   #1
mantha3 is offline mantha3  United States
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Talking No pre-Amp - Pre Amp

I have a newer Bob Latino ST-120 Tube amp. I'm digging it. Sounds great IMHO... This is my first and only tube amp so I don't have much to compare it to.

I purchased this AMP with a 21 step Attenuator. Bob and I talked on the phone when I did the order and he talked about the "The best Pre-Amp is No Preamp" school of thought. I've been running a CD player to my amp and like just having the Attenuator.

I'm wondering if anyone knows of a seller/DIY project for a componenet that would be a Attenuator with a remote control to adjust the Attenuator/volume and also have an audio source selector... So the box would have the RCA inputs... RCA output to the amp... Source selector... and then the attenuator.

I would welcome any DIY build threads and/or any products along these lines.

I'd look at a pre-amp but I think it would be a lot of features I would not use... Bass, Treble, Balance... I just don't use those and would prefer to just not spend the money on a full blown pre-amp.

Sorry for the thesis here.. I probably could have just outlined what I'm looking for. Not sure if I posted this on the right forum or not. I intend to run this on a tube amp so I put it here.
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Old 3rd February 2011, 02:51 PM   #2
6L6 is offline 6L6  United States
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Pass B1. It's like an attenuator with none of the (huge) disadvantages of running just an attenuator. Mine is for all practical purposes completely transparent.

Here is the original article - http://www.passdiy.com/pdf/B1%20Buffer%20Preamp.pdf

This is a thread showing people's completions - B1 builders thread

Mine is here - B1 preamp build thread
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Old 3rd February 2011, 03:25 PM   #3
mantha3 is offline mantha3  United States
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All these look nice. I read the article and the theory is making sense... I'd like a IR remote... At times I run my Tube Amp on my Cable TV service.. commercials and such the volume is all over the place.. So the wife (And I) are looking for a remote option as well if possible.

lazy I am!
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Old 3rd February 2011, 05:23 PM   #4
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I have to disagree with "6L6" since he has never seen or heard music through the stepped attenuator included with mantha3's ST-120 amp kit. Over 100 of these stepped attenuator's have been sold with my ST-70 and ST-120 amp kits and no one has ever complained about the quality of music heard through these amps (with their attenuators). I also wouldn't be quite so dismissive with a blanket statement like "the (huge) disadvantages of running just an attenuator".

All equipment in the "audio chain" adds some type of sonic signature to the music. This also includes the Pass B1 Diy buffer preamp that you mention as "for all practical purposes completely transparent". I think what you really mean and the key to the issue is > Can you hear any change with or without the piece of gear in the audio chain? If you can't hear a difference then why not use it. The stepped attenuator, which is an option on the ST-70 and ST-120 amp kits, was chosen after careful listening not only by myself but by other audiophiles. An Alps type potentiometer in this same situation was rejected by me for use with the amp. Although the volume tracked well between channels on the Alps clone, the amp sounded noticeably "flatter". This pot IMHO downgraded the sound of the amp noticeably more than using the amp with a quality preamp.

The stepped attenuator also includes what is basically a "pass through" of the signal on the last step so that really only a solder connection connects the input jacks to the input eyelet on the driver board. The attenuator may also be used WITH a preamp as a master level control. This is useful if the preamp being used has the volume come on quickly in the first 1/4 turn of the volume control's rotation. Placing the attenuator at 12 o'clock then necessitate more volume rotation on the preamp for the same equivalent volume through the speakers.

Bob Latino
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Old 3rd February 2011, 05:45 PM   #5
6L6 is offline 6L6  United States
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Quote:
Originally Posted by Bob01605 View Post
I have to disagree with "6L6" since he has never seen or heard music through the stepped attenuator included with mantha3's ST-120 amp kit.
This is true. Hopefully someday I can hear one - your amps seem like they should be great!

Quote:
I also wouldn't be quite so dismissive with a blanket statement like "the (huge) disadvantages of running just an attenuator".
It is my mistake by not being more specific... When I said that I was thinking of a setup akin to a "passive pre-amp" I.E., a box with a bunch of RCA jacks, switches, and a potentiometer/stepped attenuator. Those do have some big problems, mainly in the fact that you are driving way too much cable and connections with your source, and that the pot is most likely not the ideal value for all your sources and then to the power amp. I missed the part about the attenuator in the amp itself. Again, sorry.

But the OP was asking for a separate box...

Quote:
All equipment in the "audio chain" adds some type of sonic signature to the music. This also includes the Pass B1 Diy buffer preamp that you mention as "for all practical purposes completely transparent". I think what you really mean and the key to the issue is > Can you hear any change with or without the piece of gear in the audio chain? If you can't hear a difference then why not use it.
Three reasons - WAF, WAF, and overall convenience. Plugging and unplugging the sources I own that have variable outputs into and out of the amp is just not a consideration. That, and all my phonostages have fixed outputs. So having a selector and a big volume knob is rather nice.

As for listening to the sources directly connected vs. through the B1, yes, it's that transparent. Not totally, as that would be impossible, (especially as there is another set of interconnects added to the chain) but I sure can't hear a difference that is meaningful.

It wouldn't be that hard to use some off the shelf, remote control selector and attenuator on a B1

That said, I am all for attenuators properly integrated into the design of a power amp. I wish all amps had that provision!
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Old 3rd February 2011, 07:22 PM   #6
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Click the image to open in full size.

The attenuator in question is placed in the unused stereo/mono slot and is a part of the amp itself. It turns the ST-70/ST-120 into sort of an "integrated amp" and allows the ST-70/ST-120 to be used without a preamp. This works well if you only have a single high level input source and don't mind not having a remote. It is used between the input jacks and the input eyelet on the driver board. Through trial an error it was found that a 100K unit works best at least with this amp.

Bob Latino
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Old 3rd February 2011, 07:25 PM   #7
6L6 is offline 6L6  United States
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That's a nice looking install on the ST-70 chassis!
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Old 3rd February 2011, 10:35 PM   #8
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I have a similar amp - 6V6 Push Pull, it has a stepped attenuator installed on it and seems to work really well when I run my cd player straight into out without a preamp.

The one thing I do wonder, is whether it is smarter to have a unity gain buffer connected to the amp so that it is always ensuring there is a nice low output impedance entering the power amp. I assume using very short interconnects between such a buffer and the Push Pull amplifier would be important.

Is there any merit to this idea?

I was thinking of something like this:
SD-CD3

Last edited by lordearl; 3rd February 2011 at 10:40 PM.
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