Various questions from a diy noob regarding the myrefC. - diyAudio
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Old 6th June 2007, 08:06 PM   #1
thegeek is offline thegeek  Norway
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Default Various questions from a diy noob regarding the myrefC.

Hi.


First of all I'd like to warn you that I'm a noob when it comes to electronics and as such some of the following questions may be a little (to very) noobish;P

I just recently finished building a kookaburra+myref revC using kits from twisted pear audio.
I've had no big problems and as far as I can tell it's working perfectly.
So far I've only been able to test the amp with a pair of old and rather bad speakers from a philips stereo (cheap compact unit with 3cd changer++:P).

To remedy this I'm going to buy a pair of floorstanding speakers (probably 3-way), I've been looking at the new b&w 600-series (683 and 684, the 683 has fst midrange..) and Dali Ikon 6(danish speakers).
The b&w's are 8ohm, 90db spl (specs) and I'm wondering if the amp will be able to drive these or similar speakers?.
I realize the official specifications rarely present the full picture, and that speakers can be hard to drive due to impedance dips(hope that is the correct term:P), however just a general "is it worth it to try" would be nice

The amp I built uses a 300va 2x28V toroid (measured to 27Vac when connected to amp). The kookaburra uses the toroid twisted pear offer with the kit and lm4562 opamps.
Audio ground is completely seperated from power ground and ac ground. The only ground besides connections between kookaburra and the myrefs is chassis connected to ac ground. Hum is inaudible and the only noise is from the source (no noise when inputs shorted).
The amp does not get very warm, but I use pretty large heatsinks(info).

When I set out to build an amp I read up a bit all over and I kinda fell into the idea that the myref would drive even large speakers pretty well, however now that the amp is completed I've discovered that I have to turn the pot on the kookaburra to about 50-60% to get decent volume into the small speakers(6ohm, all the info I could find). This might be no problem at all, but it did make me wonder a bit if I may have acted a bit prematurely in choosing the amp:P

I've spoken to a local hifi-shop and I plan to bring the amp along to test against some speakers, but before I do so it would be really nice to know if I would be making a fool of myself if I did:P

Finally I'd really like to thank mauro penasa, the twisted pear guys and all you here on diyaudio for offering such a great service/products, it allows noobs such as myself a nice intro to the joys of diy
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Old 6th June 2007, 08:19 PM   #2
BWRX is offline BWRX  United States
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Sounds like you need to have a look at the output voltage of your source and the gains of your various components. Normally you should try to set the gain of your amp so that the output is clipping a bit with the maximum possible input voltage.

Is a kookaburra a preamp? If so, you can probably just increase the gain of the preamp instead of changing the gain of the power amp.
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Old 6th June 2007, 08:48 PM   #3
DcibeL is offline DcibeL  Canada
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The pot position on the kookaburra for decent volume is completely normal with the provided firmware. I would not worry about it one bit as long as everything is connected properly. If the signal wire is connected backwards at the MyRef amp (signal to ground, ground to signal connection), then the sound will be very low, but you should notice immediately that something is wrong because it will sound horrible as well.

If you don't like the pot position, you'll need to get a hold of a PIC programmer to remedy this. See this post, and program the PIC with the third variation. This will change how the gain values of the preamp relate to pot position.
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Old 6th June 2007, 09:00 PM   #4
thegeek is offline thegeek  Norway
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Your idea about the source is probably a really good idea. I use a dac-ah with the output bypassed (passive mod with 2.2 auricaps and 47k resistor). Bypassing the opamp is obviously going to lower the ouput, however after consulting some threads it seems that if I run the dac chips at 8V (as I do) the loss should not be very large. Either way I would really rather not have to enable the opamps again.
Your suggestion to increase the gain on the kookaburra is probably what I want, does anyone have any tips on how I would do such a thing?
I think the pga2311 + firmware in my kookaburra is set to have no gain, so I guess I would need to increase gain on the output buffer?
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Old 6th June 2007, 10:05 PM   #5
thegeek is offline thegeek  Norway
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After reading a little more I realize the buffers on the kookaburra are both unity gain, and I suppose that is not the correct place to add gain.
I think I'll test the amplifier against some speakers, if it turns out that I need to increase gain I'll do as you say DcibeL; get a pic programmer and burn the modified firmware.

Does anyone have any experience with the myref and floorstanding 3-way speakers with similar specs to the b&w's ?
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Old 7th June 2007, 02:19 AM   #6
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Default Works fine

I drove a pair of B&W 801 S2 witout any issues with a My_Ref for a while. The levels listened to were not headbanger though.
The B&W 600 series are a much easier load than the current gobbling 801's so you should be fine.
Your issue with volume is related to the dac output of your disc player. If you had a 2 volt output source the system would be plenty loud.
Hate to say it, but you most likely need either an active preamp with some gain, or an output stage after the dac. Try hooking the opamp back up to see if it solves the volume level issue.
Some people never really click with the My_Ref. I have loved all of them listened to. Just like a lot of other things, everyone has different tastes in amplifiers.
But what is even better is the hard to source Evolution also designed Mauro Penasa. It is a more refined and developed My_Ref. The one I use has paralled LM3886 chips and will drive just about anything. From high impedance, high efficiency horns to the high current 801's.



George
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Old 7th June 2007, 02:36 PM   #7
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Hello Geek, (sounds like I am talking to myself)

The Kookaburra stock has a gain of 31.5db. Thats a lot of gain...

The buffers are indeed unity gain, but the preamp as a whole is not.

The MyRef also has a pretty sizable gain of 29.5db. Together thats a lot of gain.

So you should not have any gain issues.

Keep in mind, that the volume is logarithmic (decibels are logarithmic) and that the bottom range of the kooka is -95.5db, so you have a lot of pot travel from -95.5db to + 31.5db This is normal.

Just try the speakers, I bet they will work just fine. The specs are right.

Cheers!
Russ
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Old 7th June 2007, 04:49 PM   #8
thegeek is offline thegeek  Norway
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Russ: How do I know if I have the kookaburra firmware with gain or the one without gain?

Thanks for the replies all
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Old 7th June 2007, 05:49 PM   #9
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Quote:
Originally posted by thegeek
Russ: How do I know if I have the kookaburra firmware with gain or the one without gain?

Thanks for the replies all

It comes stock with the full range of the PGA. -95.5db to + 31.5db. So unless you requested something special that is what you got.

Cheers!
Russ
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