[kelly]

I posted this question to HeadWize also but just in case someone on Head-Fi knows:

From the Stax Japanese web site:
http://www.stax.co.jp/ENG/SRM-717-E.html

"The STAX SRM-717 provides a bypass mode that enables "direct" connection to preamplifiers, passive volume controls or integrated amplifiers and receivers with "pre out' facilities."

Does anyone know if engaging this mode effects the gain of the amplifier? According to the specs and Kevin Gilmore's helpful interpretation, the maximum voltage of the SRM- 717 is 1V RMS. Most preamps have considerably more voltage (some up to 12V. The purpose of the bypass mode, of course, is to bypass the volume pot and drive the amplifier directly from a preamp. I need to know with certainty what voltage the Stax amp is expecting for this purpose.

If anyone has the amplifier and can consult the manual or is willing to switch it into bypass mode and plug it into even a 2V line level source to see how loud it is, I'd be most appreciative.

 

[Jupiter]

You could always email them at info@stax.co.jp and you should hear back from them within a day.

 

[KurtW]

According to the data sheet that you linked, "Rated Input Level 100mV/100V output, Maximum Output Voltage 450Vr.m.s." This agrees with the spec that it has 60dB gain. This means you only need 450mv to drive it to its maximum output. I would doubt very much that the bypass mode affects the gain; there is no reason it should, and if it did they would probably say.

You would be driving the Stax in bypass mode with a preamp with a volume control, so as long as it had at least 450mv output you should be fine. Most amps can be driven to full power with around 1 to 1.5 volts, some less, so most preamps are set up to have useful volume control settings up to this value and lower. A preamp may be able to put out much higher voltage if the input voltage is high enough, but I don't know any amp that requires more than 2 volts for maximum power and most are less.

The good news about the sensitivity of the Stax is that most CD players put out more than enough voltage to drive it to full volume, so you can get by with a passive preamp if you want, as you don't need gain. In fact Stax even mentions the use of passive preamps when using the bypass mode.

 

[kelly]

Jupiter
Kurt

Thanks for replying.

I think you're right that the amp will be more than satiated by a standard 2V output and passive preamp but I want to be sure that I have enough play in the volume pot so finding its maximum is important. I emailed Stax and replied again today to try to find out the maximum input voltage.

My reason for this consideration, of course, is that my CD player is being modded with a tube output stage and "amp-direct" mode that controls the voltage to the tube stage via a stepped attenuator. I'd ideally like to control both the headphone amp and a speaker amp from this stepped attenuator. Modwright offers to set the output voltage to anything you like, up to 12V. With only 24 steps on the attenuator, if I opted for 12V and the maximum input voltage on the amp was only 2V, I'd only be able to listen using the very first click on the stepped attenuator, and even that may be too loud.

My hope is that while he offers the higher voltage settings that most modern amps won't need it and that if I end up asking for 0-2V that it will be sufficient for whatever speaker amp I end up getting as well as giving me enough play in the volume for the Stax.

I'll wait for Stax' reply before making a final decision. If you have anything else, please reply again soon.

 

[KurtW]

Again, I don't know any amp that needs more than 2v for full power, and most are less, even old ones. For example, the Classe CA201 needs 1.3v for 200 watts per channel output, and I don't think most people will want it to do 200 watts very often. I don't think you'll want to drive the Stax to 450v often either, but I'm sure you'll want that capability. So you'll want some decent control in the 100mv range for it.

An important consideration for your stepped attenuator is not only the number of steps but also the change per step and the total range. The change in steps in often variable, giving bigger steps at each end and smaller steps in the range where you would normally use it. Also, it won't be linear in terms of voltage but more of a log relationship. If you can also specify the step sizes, then you should find out what the sensitivities of all of the equipment that you might be using it with and come up with a comprimise if necessary...which sounds like what you're doing. You might want to give this info to Modwright and have them recommend the steps and range.

 

[kelly]

Kurt
Modwright is using the DACT which I believe as he's using it, just divides the voltage supplied to the tube stage. I don't know that doing anything other than linear would be an option there. (At 12V, it'd be .5V per step! What amps use this??)

Thanks for the info on the Classe since so far that's my favorite amp in the prices I'd be able to afford for the wattage I'd want. (I know, I know - it's tragic to be attracted to inefficient speakers--I could never afford the wattage I need in a good tube amp).

 

[KurtW]

According to DACT, the 24 output levels are: 0dB, -2dB, -4dB, -6dB, -8dB, -10dB, -12dB, -14dB, -16dB, -18dB, -20dB, -22dB, -24dB, -26dB, -28dB, -30dB, -32dB, -34dB, -38dB, -42dB, -46dB, -50dB, -60dB.

dB = 20*log V2/V1

Therefore, a 12 volt input attenuated by 50dB would give you 38mV, by 60dB is only 12mV. Still, that one step jumps the voltage up by 3X.

As you can see, its 2dB per step down to -34dB, then 4dB per step except for the last step being 10dB. So you want your normal range to be in the 2dB step area, or less than -34dB attenuation, which equates to a 50X voltage reduction. 12v/50 gives 240mV which would probably be at the loud end for the Stax, since you only need 100mV for a 100dB SPL loudness. 2 volts reduced 34dB gives you 40mV, probably a pretty reasonable number.

 

[kelly]

Kurt
Maybe I'm confused on this distinction, but isn't the DACT normally used to attenuate the output? That seems to be what you're representing.

Modwright's DACT is placed *before* the tube stage and is used to divide the voltae prior to the tube stage amplifying it. So, even if that formula is true, it's on a different scale. Isn't it?

 

[KurtW]

The DACT doesn't know or care about where its placed, its doing to do its thing the same way. If there is another stage after the DACT, its going to be a fixed gain. Deciding on attenuating the input or the output of this stage has some implications on the circuit, but since dB is a ratio of voltages, it all works out the same. Think of it this way: 10 times 0.5 = 0.5 times 10.

 

[kelly]

Kurt
I think you begin to understand why I chose the Mass Comm. major over the EE one. Thanks for your continued patience.

I read your previous post about eight times. You are saying what you would ask Dan for in my shoes is a maximum output of 2V, right?

 

[KurtW]

If I were you I would discuss it with Dan just in case there is something missing from what we've been discussing, but it seems like 2v would be good. I'm assuming that this number is the output voltage from a typical CD. If it means something else, like its the maximum voltage from a test CD and typically CDs are only half of that or something, then that needs to be taken into account. Remember its always better to have a little more gain than you need than not enough.

 

[kelly]

Kurt

I've been discussing it with Dan and forwarding him stuff from you and from Stax. Since Gilmore said 1V would be sufficient and looking at your math and knowing the typical CD player line out (that can usually drive an amp with a passive preamp) is 2V, I'm thinking that's going to be the way it'll happen. I'll wait for Stax's next reply, though.

If I had the money to go ahead and get the Stax, I'd just forward it to Dan, but the way things worked out he'll be finishing up my player any day now and I won't have the funds for the Stax just yet.

Ah just think, before long I may actually have to start bugging you guys about cables.

Thanks a lot, Kurt.

 

[kelly]

Well, I thought we had it figured out. Here's what Stax replied with:
"Thank you for your question.

RE : Maximum input level,

The maximum output voltage of SRM-717 is 450V r.m.s /1 KHz (for Earspeaker load).
Therefore, the maximum input voltage in "a bypass mode" becomes about 0.45V r.m.s. because the gain
of SRM-717 is 60 dB."