Objective2 as preamp (Help Needed) - diyAudio
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Old 7th September 2012, 06:08 PM   #1
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Default Objective2 as preamp (Help Needed)

Hello,

I'd like to mod my Obj2 to be compitable as a basic preamp to my Parasound ZampV3. The Zamp has 33kohm input, FWIR the optimal impedance ratio pre-power is 1:10 so tha means i need a 3.3Komh output off the Obj2. My theortical Electronics level is very minimal So i need a detailed insturuction how to perform this (admittedly plain) mod: What resistor values do i need (simply 3.3Kohm?) and at what point do i place them and whether i need to change anythiung elese in the circuit.

Thanks for any help.
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Old 7th September 2012, 06:28 PM   #2
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You don't need 3k3 out of the O2, you can leave it unchanged. Better than 1:10 is not a problem

The O2 is a quiet amp, but whether it is quiet enough for a phono preamp is another question. The other potential issue is RIAA equalisation if you intend to use vinyl.

What sources do you intend to use?
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Old 7th September 2012, 08:03 PM   #3
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I did try connecting as is but the result is much poorer dynamics and a very noticable ditortion going in the bass.

My source is a DAC (CIAudio VDA-2) which outputs 2Vrms SE and i'd just like to boost the input signal to the Zamp as it's apperantly not enough voltage swing for the amazingly inefficient (vintage) Sony ECR-500 electrets.
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Old 7th September 2012, 11:50 PM   #4
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The Parasound spec says 1.5V for full output. It doesn't say Pk-Pk, peak, rms or what. 2Vrms is 5.6V Pk-Pk. With the gain of the O2 you may simply be overloading the input.

Try a simple volume control before the parasound (omitting the O2), built with a 5k dual log pot. Dual log 5k are a bit thin on the ground. If you don't mind waiting a few days you can get 5K pots cheap here:- 2 x 5K OHM Logarithmic Dual Rotary Taper Potentiometer A5K 5KA POT ALPHA | eBay

... although you'll have to buy 2. A 10k pot will do at a pinch.

If the ECR-500s are inefficient, you can't compensate by adding gain at the front end, it won't get you an undistorted increase in voltage at the output of the Parasound.

Last edited by counter culture; 7th September 2012 at 11:55 PM.
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Old 8th September 2012, 03:10 PM   #5
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Thanks foy your suggestion, but the Zamp already has attenuation knobs at the back. I've tried reducing it and playing with the obj2's as attenuator but it sounds much better when the Zamp is maxed and the obj2 at around 10-12 o'clock (gain is stock 2.5). Still, not much increase in volume and as i said sounds quite poor.

So you're certain a 3.3K out won't be beneficial? Should i look for another speaker amp? Thing is, size is a major factor for me and i don;t know anything more powerful than the Zamp for it's size.
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Old 8th September 2012, 04:37 PM   #6
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Yes, I'm certain 3k3 out won't help.

I found some more stuff on the Sonys, which I didn't know much about.

Best I can discover (in a thread on head-fi) the EDR-500's driver box presents a 4 ohm impedance to the amplifier. Most headphone outputs have a limited drive capability, and most dedicated headphone amps will not drive this load either, if in fact 4 ohms is accurate. You could try driving the box direct from the amplifier outputs.

The ECR-500 spec that I found says 90dB SPL (quiet) for 1V in. Th Zamp will drive 4 ohms to 60W which is ~15Vrms which is 23dB over 1V(rms) which is still 7dB below the ECR-500's max rated 120dB SPL. Make up an adapter with a stereo jack socket or whatever is necessary attached to the speaker output of the Zamp, start with the volume low and turn it up slowly.

Quite honestly I'd be nervous about 60WPC disappearing into a little box, but if you start with the volume low you shouldn't destroy anything. I wouldn't drive the Zamp at full volume into 4 ohms for any length of time. Although it's rated for it it may get pretty hot depending on the type of design, better to be cautious and leave something in reserve.
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Old 10th September 2012, 07:12 PM   #7
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See, the funny part is i already hooked them to the speaker outs right from the begining and i assure you that it;s nowhere close to 113db or even 90db, Hence me describing them as amazingly inefficient. But the amp is nowhere near hot at it's max gain driving them, which is odd since it should be if it's indeed outputting 60Wpc into the load.

Anyway, I guess i should try them out of an even meaner machine to see if they finally open up or else part ways.

Thanks again for your help.
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