Spiug31 Posted March 26, 2009 Report Share Posted March 26, 2009 I'd appreciate some advice here. My mini-dacs' xlr output volume is too high for comfortable listening. I intend to make use of attenuators/voltage dividers of type U (shown below) to manage the volume to a comfortable level. That I've seen here I've done some testing and the value of resistance that works best for me is an attenuation of 30dB (68K ohm on both pin 2 and 3 (the R1 positions)). The value of resistance I measure between pin 2 and 3 on my mini-dac when it's unpowered is 4450 ohm. I am presuming this is the source impedance and should be matched by the shunt resistance (shown as R2). My 4-channel rockhopper beta22 is fitted with two dact CT2 attenuators. I am attenuating the signal at the interconnect cables' xlr connector just prior to it entering my amp. Question: As these resistors are mostly in the signal path what types would you recommend me using where ? n.b. I have adjusted the mini-dacs' inbuilt trimpots (which has helped) but it is still too loud for easy listening without attenuation. It'll be a while (over six months) before I can afford to change source but I am willing to consider the expense of resistors such as the TX2575 for use at points in these attenuators, if they will give the best result. In the short term I am using Kiwame 2W resistors at R2 and have both shinkoh and kiwame values here for the R1 positions. Quote Link to comment Share on other sites More sharing options...
Pars Posted March 26, 2009 Report Share Posted March 26, 2009 Can you adjust the gain of the b22? Amb shows R3/R4 values for a gain down to 2 (which is as low as it can go)? I would think that would be better if possible. Otherwise, yeah, you will want to use good resistors. There are some resistors in the amp that are also in the signal path, so I'm not sure that going really special here (TX2575s, etc.) will buy you that much over something like Dales or PRPs. Quote Link to comment Share on other sites More sharing options...
grawk Posted March 26, 2009 Report Share Posted March 26, 2009 Pretty sure there are jumpers you can reduce the output on the xlrs inside the minidac. Check the manual. Quote Link to comment Share on other sites More sharing options...
morphsci Posted March 26, 2009 Report Share Posted March 26, 2009 The jumpers determine if the xlr output is affected by the internal trimpots and the front volume control or only the trimpots. Quote Link to comment Share on other sites More sharing options...
Spiug31 Posted March 26, 2009 Author Report Share Posted March 26, 2009 The jumpers determine if the xlr output is affected by the internal trimpots and the front volume control or only the trimpots. yup, what he said the sound gets noticably blurred by using the minidacs front volume control instead of trimpots + fixed value resistor attenuation. my beta22 has R3 and R4 as 2491 which reading from the schematics means it is set with a gain of 2. The resistors used are printed with Dale RN55D. Running a 1kHz 16bit 44100hz tone via toslink from my H120 to the minidac generates a minimum voltage out (pin 2 to pin 3) of 3.352V left channel and 3.346V right channel so I`ve adjusted them to both be 3.352V. This is what influenced me to think that attenuation or a new source is needed. I`ll check around for the price of RN55D resistors over here. Quote Link to comment Share on other sites More sharing options...
cetoole Posted March 26, 2009 Report Share Posted March 26, 2009 This may be a more involved modification than you are looking for, but you could just look into changing out the I/V resistor used in the MiniDAC. Pretty sure this uses the AD1955, and has a pretty standard opamp I/V, so should be fairly easy to find this resistor and simply replace it. True, you would be making a fairly large change, so should also change some of the LPF components, but nothing too radical. Beta22 isnt stable below a gain of 2, right? Off topic, but those Donuts doin OK for you? Quote Link to comment Share on other sites More sharing options...
Spiug31 Posted March 27, 2009 Author Report Share Posted March 27, 2009 This may be a more involved modification than you are looking for, but you could just look into changing out the I/V resistor used in the MiniDAC. Pretty sure this uses the AD1955, and has a pretty standard opamp I/V, so should be fairly easy to find this resistor and simply replace it. True, you would be making a fairly large change, so should also change some of the LPF components, but nothing too radical. I`m not comfortable desoldering/soldering components on that cramped a board (requires too much coordination, balance andprolonged visual focus for me). Beta22 isnt stable below a gain of 2, right? gain of 2 is lowest I`ve seen mention of. Off topic, but those Donuts doin OK for you? they have eased my muscles on several nights, as far as audio impressions go they are near perfect but for my taste could do with a little more visceral/bite, wetter sound, solid imaging, denser blacks and colours. In other words there is a slight veil at present. this may I hope resolve itself once I finally get my amp, source and cable to play ball Currently sonic presentation changes a lot with position/angle/height of the cups which I hope will also be resolved in time / with practice. I can see myself missing the donuts and having pangs if I were to sell them. The HD650`s I find are more an intelectual exercise than a part of me, I percieve a cold void in their sound which combined with a rolloff isn`t pleasant. Quote Link to comment Share on other sites More sharing options...
Pars Posted March 27, 2009 Report Share Posted March 27, 2009 I`m not comfortable desoldering/soldering components on that cramped a board (requires too much coordination, balance andprolonged visual focus for me). <snip> Since that DAC is SMD, it might be rather easy to just parallel a resistor on top of the current I/V resistor. Figuring out which is the I/V resistor might be somewhat fun, though it would be on the first opamps. It would need to be lowered some in value to decrease the output level, so paralleling a 10K or something might be good. Lowest gain on the beta is 2, so no joy there. Quote Link to comment Share on other sites More sharing options...
Spiug31 Posted March 27, 2009 Author Report Share Posted March 27, 2009 Figuring out which is the I/V resistor might be somewhat fun fun as in interesting, pedantically fun or fun as in smoking components ? here is a big pic of the mini-dac board http://www.head-case.org/forums/members/spiug31-albums-apogee-mini-dac-internals-picture612-big-pic.jpg how would I identify the correct resistor ? n.b. my multimeter is a tenma 72-1016 (ebay buy ) - off topic - my previous listening impressions are no longer current as battery powering the minidac and plugging the beta22 plug into the wall rather than the extension cable have changed the sound again. Quote Link to comment Share on other sites More sharing options...
HeadphoneAddict Posted March 27, 2009 Report Share Posted March 27, 2009 What changes did you hear by moving the mini-DAC to battery power? I use a Sigma 11 PSU (that n_maher built) with my mini-DAC. The highs seemed to get smoother and more refined with it, but gain or output volume didn't change. Quote Link to comment Share on other sites More sharing options...
Spiug31 Posted March 27, 2009 Author Report Share Posted March 27, 2009 What changes did you hear by moving the mini-DAC to battery power? I use a Sigma 11 PSU (that n_maher built) with my mini-DAC. The highs seemed to get smoother and more refined with it, but gain or output volume didn't change. think back two months http://www.head-case.org/forums/headphone-amplification/5358-apogee-mini-dac-hp-out-vs-headfive-via-xlr-rca.html#post196203 Quote Link to comment Share on other sites More sharing options...
n_maher Posted March 27, 2009 Report Share Posted March 27, 2009 The essential problem here is that balanced you're probably getting 3 to 4V out of your source and the amp's gain is effectively 4. With efficient headphones that's going to yield very little adjustment range. Quote Link to comment Share on other sites More sharing options...
luvdunhill Posted March 27, 2009 Report Share Posted March 27, 2009 Beta22 isnt stable below a gain of 2, right? I'd assume it is, because one can build a ground channel. 6 channel beta22 buffer FTW! Quote Link to comment Share on other sites More sharing options...
Pars Posted March 27, 2009 Report Share Posted March 27, 2009 From amb.org: Please note that in the 3-channel "active ground" configuration, some part values in the ground channel differ from that of the left and right channels. See the following table: Part Left & Right channels Ground channel R1 475Ω (not used) R2 1MΩ 1KΩ R3 (see table below) 2.49KΩ R4 (see table below) 2.49KΩ C2-C5 (see table below) 100pF The following table shows the left and right channel part values when building the β22 with a non-default voltage gain (gain of 8x is the default): L & R Part Gain 11x Gain 8x Gain 5x Gain 2x R3 100Ω 182Ω 392Ω 2.49KΩ R4 1KΩ 1.21KΩ 1.5KΩ 2.49KΩ C2-C5 22pF 33pF 47pF 100pF These values were carefully tuned for the an optimal combination of speed/bandwidth and stability. The β22 should not be operated with a gain of less than 2x. Quote Link to comment Share on other sites More sharing options...
Spiug31 Posted March 27, 2009 Author Report Share Posted March 27, 2009 Having had a proper look at the size of components, lack of clearance around the resistors on the minidac board and the difficuilty for me to get good lighting/magnification of the work area I`ve decided not to adapt it. Instead I`ll finish the idea to make do with applying a voltage divider just prior to my amps inputs to tide me through and will look forward to an upgrade of source later on. The essential problem here is that balanced you're probably getting 3 to 4V out of your source and the amp's gain is effectively 4. With efficient headphones that's going to yield very little adjustment range. This minidac provides an unusually hot XLR signal, following the instructions in the manual (quoted below) the minimum voltage I get is 3.352V. Calibration – From the factory, the Mini-DAC is calibrated for a maximum analog output level of +20 dBu, or –16 dBFs = +4 dBu (at the XLR ANALOG OUT connectors with the LEVEL knob fully clockwise). To re-calibrate to another conversion level, follow these steps: 1) Turn the LEVEL knob fully clockwise. 2) Input a 1kHz sine wave at the desired digital reference level (most likely between –20 and –12 dBFs). This may be most easily accomplished by connecting the Mini-DAC to a computer running an application capable of generating digital reference tones. 3) Turn trimpots R55 and R 71 until a +4 dBu signal is obtained at the XLR ANALOG OUT connectors. A + 4 dBu output level may be accurately determined with even the most inexpensive voltmeter; simply measure between pins 2 and 3 of the XLR ANALOG OUTPUT connector for an AC voltage of 1.23 volts. Applying the attenuation should solve that in an easy to reverse manner. I`m knackered now, thanks for all the knowledge, fare well Quote Link to comment Share on other sites More sharing options...
n_maher Posted March 27, 2009 Report Share Posted March 27, 2009 Are you saying your unable to lower the output voltage by following those instructions? If so, I'd look into solving that problem before building the new attenuator. Quote Link to comment Share on other sites More sharing options...
HeadphoneAddict Posted March 27, 2009 Report Share Posted March 27, 2009 think back two months http://www.head-case.org/forums/headphone-amplification/5358-apogee-mini-dac-hp-out-vs-headfive-via-xlr-rca.html#post196203 Thanks for the reminder - keep forgetting sla = sealed lead acid battery I would think most DAC with XLR out will still give you higher levels than you want, and maybe you need a preamp instead of a new source, or an amp with less gain? Quote Link to comment Share on other sites More sharing options...
Voltron Posted March 27, 2009 Report Share Posted March 27, 2009 SLA = Quote Link to comment Share on other sites More sharing options...
Spiug31 Posted April 22, 2009 Author Report Share Posted April 22, 2009 This is what I`ve now got fitted (in an easy to remove manner) and it seems to be doing the job. I`ll get around to sorting the sources problem when I`m able but for now this is a working patch. Quote Link to comment Share on other sites More sharing options...
Spiug31 Posted April 27, 2009 Author Report Share Posted April 27, 2009 I was wrong about my mini-dac being unusually loud. Having made audacity generate a 1kHz tone at -12dB and fed this to the mini_dac via usb I am now able to adjust the trimpots to each give a voltage of 1.230V (+4dBu) . It seems to simply be the case that +4dBu is to loud an output for me to use with normal files. I`ll be keeping the resistors in place on the beta22 until I can get a source with lower volume output. silver lining here is the mini-dac is working fully. This minidac provides an unusually hot XLR signal, following the instructions in the manual (quoted below) the minimum voltage I get is 3.352V. Quote Link to comment Share on other sites More sharing options...
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