[JustAudio]

Hi anyone tried modwright with emotiva xpa2 amps? the input impedance of the emotiva is relatively low at 23.5kohms. would this cause roll off issues? Thanks

JA

[wilsynet]

Modwright LS 100 has 300 ohms output Z.  Multiply by 20 gets you to a minimum required input Z of 6000 ohms.  Your amplifier has over 20K ohms input Z.  That's plenty of headroom so I expect you should be fine.

[Rainier42]

I'd be careful ... many preamps output impedance increases dramatically in the lower frequencies, if this is the case with the LS 100, you would experience roll off.  I have an XPA-2 as well and have considered carefully which tube preamp to purchase due to this.  Stereophile is a good source for these measurements.  Unfortunately, the LS100 is not one they've tested.

[modwright]

I will measure our LS 100 at low frequencies and determine if this is the case.

The other thing to consider is the size of output coupling cap that the tube preamp uses.  We use 4.7uf, and this value, combined with the input impedance of the amp forms the high-pass filter that CAN lead to LF roll-off.

For instance, using F = 1/2*pi*X*C

For an input impedance of X =15K (our amps for instance), C = 4.7uf output coupling caps, gives a -3dB point of 2.23Hz.

Compare this to a tube preamp that uses 1uf output coupling caps, where the -3dB point is 10Hz and consider that this is a gradual slope, so there WILL be rolloff at 20Hz.

Thanks!

Dan

[SoCalWJS]

I'll follow this thread as well - I have the XPA-1's, so I'm curious.

Sounds like that IF there is a dropoff, it would only be a db or 2 at a very low frequency (below 10 hz or so?) if I'm reading this correctly.

My measured in-room response at 20 hz is reasonably close to flat, so I'm not too worried unless there is a different issue.

[modwright]

Well, what is important to acknowledge, is that the -3dB down point means that it starts dropping at a higher frequency and it is a first-order curve, so not a sharp curve.  If the -3dB down point is in the 2Hz - 3Hz range, then yes, there would be no more than 1-2 dB drop at 10Hz.

I would not worry, based on the input impedance of 23.5K and the fact that we successfully drive our 15K (Z-in) amps of our own, I would NOT worry about loss of bass!

Thanks,

Dan W.