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MiniDSP SHD and old Mac1900

graphical

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Hi all,

I'm looking for a new amplifier (maybe) but I'm trying to match everything correctly because I'm finding that the MiniDSP SHD and Mac1900 combination is not giving me the gain I want.

The setup. QOBUZ -> MiniDSP SHD through "Amp In" inputs to the Mac receiver's amplifier. With Dirac off, I'm not getting the gain I want. Speakers are Magneplanar MC-1 with 2 DWM. I have the third and fourth channels from the DSP routed to an Aiyima A07 for the time being to drive the DWMs. I plan to buy a single four channel amplifier. The Aiyima has the same problem as what I will describe. Having three separate volume controls is not a long term solution.

The Mac1900 is a roughly 1974 model that I've loved for 20 some odd years. It has been refurbished within the last two years and is pretty well on spec. Output spec is 55WRMS / channel (20-20kHz @ .1%THD). Output spec is also Output spec is 29.98V @ 8Ohms. All the Mac specs seem to be RMS but a straight calculation there would give me 112W. I know that Mac amps are famous for being underrated, but that seems severe. What am I doing wrong?

Input spec for the amp in is 2.5V @ 100kOhms impedance. Output spec is 29.98V @ 8Ohms. That suggests a gain of 21.6dB. Notably, the Magneplanar speakers are not nominally 8 Ohms. They are spec'ed at 4 Ohms. The Mac1900 manual says the power output is the same (55WRMS) at 4 or 8 Ohms. For 16 Ohms, they spec the receiver at 30W.

The SHD unbalanced output spec is 2.0V RMS. I assume that's at 0db on the volume attenuator, but the SHD software allows me to increase the output level to 12dB. Is the 2.0VRMS with everything set at 0dB?

My problem is that the volume I want (most of the time) requires me to set the SHD volume to around -10dB which is the max setting when running Dirac. I want more dynamic range.

One solution that works is to send the output of the MiniDSP SHD into the Tape In on the Mac1900. These are spec'ed for 250mV input. I can use the DSP on the SHD to reduce the output by 18dB and then I should have the right input voltage, correct? 20log(250/2000) = -18dB. Interestingly, the manual gives the spec for gain in the preamp - 20dB. And that makes sense: 20log(2.5/.25) = 20db.

This works but I still don't get the gain I need unfortunately. I haven't found the optimal setting yet for the miniDSP but -18dB isn't enough if I keep the volume set at -10dB. I'm pretty sure the Mac1900 is fairly robust, but I can saturate the tape input, I think.

Granted the big problem is the Magnepan speakers which require more power than I'm giving them. I get all the volume I need, but I have to raise the SHD to 0dB which leaves me limited headroom. I have other speakers that do not require this much work, so I know the Magnepans are inefficient. I do like them and I like the Mac1900 but something will have to give.

Question. How much gain do I need? What amplifier is well-matched to the miniDSP SHD's 2.0VRMS output? I prefer Class D and don't plan to spend more than $2000.

One potential test. Amir shows the Aiyima A07 at 29dB gain. Aiyima's spec on their website is 21.5dB. That's a significant difference. So at some level, putting the Aiyima in as the main amp to see what happens is only valid if I know that number. Where's that dang heat sink again?

Thanks, John
 

mdsimon2

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Hi all,

The SHD unbalanced output spec is 2.0V RMS. I assume that's at 0db on the volume attenuator, but the SHD software allows me to increase the output level to 12dB. Is the 2.0VRMS with everything set at 0dB?

SHD will never output more than 2.0 V from the unbalanced outputs. However, you can get there multiple ways. If you send a 0 dBFS signal to the SHD then you would reach 2.0 V at a volume position of 0 dB. If you apply boost via Dirac, general output level adjust or PEQ you can also get to a 2.0 V output signal at a lower volume position. I would watch the output meters to see how close you are to 0 dB, the level of the recording matters a lot here.

My problem is that the volume I want (most of the time) requires me to set the SHD volume to around -10dB which is the max setting when running Dirac. I want more dynamic range.

I would measure the actual boost you have applied in Dirac before worrying about cutting 10 dB. Again, level of recording matters here, if you aren't getting close to 0 dB on the output meters at -10 dB volume position you can safely turn it up.

One solution that works is to send the output of the MiniDSP SHD into the Tape In on the Mac1900. These are spec'ed for 250mV input. I can use the DSP on the SHD to reduce the output by 18dB and then I should have the right input voltage, correct? 20log(250/2000) = -18dB. Interestingly, the manual gives the spec for gain in the preamp - 20dB. And that makes sense: 20log(2.5/.25) = 20db.

This works but I still don't get the gain I need unfortunately. I haven't found the optimal setting yet for the miniDSP but -18dB isn't enough if I keep the volume set at -10dB. I'm pretty sure the Mac1900 is fairly robust, but I can saturate the tape input, I think.

Using the Mac1900 preamp certainly will give you 2 dB more output than the SHD is capable of by itself, although limiting the SHD output to 0.250 V does not seem like the best idea from a SNR perspective. Again, I wouldn't worry too much using a fixed cut of 18 dB, just look at the output meters and see how close you are to -18 dB.

I assume you are doing volume control in the SHD? If so I would probably just turn it up and not worry about it. If you start thinking you are clipping the SHD outputs I would then check the level meters to see if you are getting above 0 dB (or -18 dB if you are using the Mac1900 preamp).

Michael
 

abdo123

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Question. How much gain do I need? What amplifier is well-matched to the miniDSP SHD's 2.0VRMS output? I prefer Class D and don't plan to spend more than $2000.

Any amplifier with balanced inputs with a gain of ~20 dB or higher would work great.

This means any Hypex based amplifier would work great.
 

Godataloss

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Just took delivery of my Buckeye Amp this weekend and I have to say I'm exceedingly impressed with the pairing of my Minidsp SHD. It is dead quiet on my 100+db speakers. As a vintage guy and owner and lover of various tube amps, I gotta say there is a lot to like about an amp that performs like this without any drama. I left the whole system on all weekend log and it never even got warm to the touch. Admittedly, my speakers are much more efficient than yours, but I think the higher current Hypex modules would serve your needs well.
 
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graphical

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Thanks for the advice. This is all very helpful.

@mdsimon2 I agree completely that running the SHD into the Tape In is a hack, but I did it. The first time I tried, I amplified noise. After some work to optimize everything, I got it sounding good, but then someone blew on it and knocked that fragile house of cards down. I will watch the meters on the output and see how close I really get to 0dB and then try to optimize. Also a good idea to check to see how much gain/loss Dirac is providing.That's a helpful idea. Thank you.

@abdo123 Yes, the balanced input goes to 4V, but that's balanced so at the input, the circuit will reject the common mode and deliver a signal voltage to the next stage. I don't know how we measure. I guess I need to watch Amir's video. Does using the balanced input increase the voltage delivered to the amplifier or just reduce the noise? Looking at Amir's tests of different amps, it appears that he adjusts the voltage to give 5.000V output. The numbers vary greatly. In any case, Amir's Buckeye review of the NC252MP shows 25.7dB gain, so per your advice, that should work great. Thanks.

@Godataloss Glad you're enjoying Buckeye. Did you buy the 252 or the 502? I assume my speakers would be better with more power, but the 252 seems plenty powerful. I do keep a couple of tube preamps around, but frankly, the old Mac1900 has been the rock solid performer for three decades for me. But the Maggies seem to need a little more. Enjoy the music!

Thanks to you all.
 

beren777

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You could buy a VTV Hypex NC252MP to try out. Buckeye is currently backed up until January anyway. I have one and while there are implementation details I wish were different, it works fine. If it doesn't power your speakers well enough, then bump up to a Hypex 502 model.
 

Godataloss

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Thanks for the advice. This is all very helpful.

@Godataloss Glad you're enjoying Buckeye. Did you buy the 252 or the 502? I assume my speakers would be better with more power, but the 252 seems plenty powerful. I do keep a couple of tube preamps around, but frankly, the old Mac1900 has been the rock solid performer for three decades for me. But the Maggies seem to need a little more. Enjoy the music!

Thanks to you all.
I purchased a stereo 252 based solely on Amir's review and the fact that they're made 'just up the road' from me. Amazed at the performance level for $500 delivered. I would spring for the 502 with the Maggies if I were you.
 
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graphical

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@Godataloss Will do. I'm a little tempted by the Purifi, but I figure the difference between the 502 and the 1ET400 is not going to be noticeable unless I upgrade the room:)

@beren777 Yeah. I've been through the exchange hassle with one company who shall remain nameless until we have to escalate by making public the story. Not feeling like trying something out again because of that.

@abdo123 Thanks. Then switching to balanced would give me that "perceptible" change in volume. If I wind up putting the old amp on the scope to measure these things, I'll be entering geek mode again. Hobbies are good.
 
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