To preamp Or Not to

Jrp27

Member
2016-04-22 7:27 pm
I recently built a preamp. A clone of nac152XS.
After connecting the preamp to the amp thesound from the speakers appear less dynamic.

When I connect the amp directly to the iPod. The sound is better. So the preamp maybe limits the sound and dynamics.

Also if I increase the volume on the preamp. The bass notes appear distorted.

Wondering why is this happening? Do you really need to use a preamp even your audio source is a mobile phone or iPod?
 
No you don't actually need a pre-amp unless you are using multiple inputs and don't want to be constantly changing patch cords.

But... after all the work building it, it would be a shame to simply set it aside...

It could be that your iPod is over-driving the pre-amp or the combination is overdriving your amplifier. Try turning down the iPod's output level to about half and see how it sounds. If you amp has an input gain (volume) control try turning it down a bit. If possible try it with a different source, like a DAC, CD player or Tuner and see how it sounds.

From your description it sounds like you may also have a couple of minor problems, perhaps bad solder or a faulty part. A bit of time double checking your work might prove very helpful. (I've been at this a long time and I still mess up the occasional solder joint.)
 
Perhaps the phone is overloading the pre, and as I hate cells, I don't know exactly their audio quality and levels, but it appears from your words, to be useless.

Phones and iPods would qualify as "Mid-Fi". The sound quality is pretty good but the output levels on the headphone jacks are often very high ...

The iPod in particular was designed as a blaster box, intended to drive earphones to a level that would leave you dizzy.
 

Jrp27

Member
2016-04-22 7:27 pm
No you don't actually need a pre-amp unless you are using multiple inputs and don't want to be constantly changing patch cords.

But... after all the work building it, it would be a shame to simply set it aside...

It could be that your iPod is over-driving the pre-amp or the combination is overdriving your amplifier. Try turning down the iPod's output level to about half and see how it sounds. If you amp has an input gain (volume) control try turning it down a bit. If possible try it with a different source, like a DAC, CD player or Tuner and see how it sounds.

From your description it sounds like you may also have a couple of minor problems, perhaps bad solder or a faulty part. A bit of time double checking your work might prove very helpful. (I've been at this a long time and I still mess up the occasional solder joint.)

At lower ipod volume it still is ok. But increasing the volume again distorts the sound.

When I touch the bare input leads ( preamp) I get a reduced hum lesser than what would happen if I touch the power amp inputs.
 
At lower ipod volume it still is ok. But increasing the volume again distorts the sound.

iPods have a pretty strong output on them. It's no surprise it's overloading your pre-amp inputs. So your best bet is to keep the iPod turned down and manipulate your system with the pre-amp's volume control.

As I mentioned, if your power amp has input controls on it you may want to keep them down a bit as well.
 
Could be that the input impedance of the preamp isn't well suited for the iPod output, it occurs to me.

Perhaps building a small impedance-dropping attenuator would do the trick? You know, 2.2 kΩ and 470 Ω in series from input jack 'hot' to ground, with a tap of an 'attenuated hot' at the midpoint.

Attenuation is –15 dB, which ought to tame the iPod's output considerably. Moreover, the 2.67 kΩ series-sum load is probably perfect for the iPod. The midpoint presents a 388 Ω 'source impedance' to the preamp.

Since the rule is “low drives low-to-mid-to-hi, but hi-can-only-drive-hi” impedance inputs, there is no issue with a 388 Ω source driving an expected 10 kΩ to 1,000 kΩ preamp input.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
 
Is the volume control located before all the active circuitry? If so, the preamp circuit cannot be overloaded before its output clips.

point well made, tho' I'd just make the side comment that there are too many 'knobs' involved.

(virtual knob) inside the iPod, scaling the output DAC
(real knob) inside the preamp, feeding unknown gain stage
(real knob) inside the amplifier, as is pretty conventional.

For reasons that are fairly obvious to anyone having-been-in-audio-for-awhile, it is ostensibly 'best' when whatever-the-source's internals are “set to '10' ”; this way, the signal is at the maximum relative to the baseline noise of the thing.

The input pad of preamplifier stage … in turn … then can cut that, and whatever relative noise is part of the same signal, by whatever proportion is desired.

However, the preamplifier's output ought to be at its reasonable optimum to allow for substantial signal dynamics beyond what one might naïvely expect from the “RMS” output. 5× is not unreasonable. 10× is even more research-validated.

That in turn then is input to the main amplifier. IF the amplifier has no input pad, then it is the preamplifier that really must have an output-potentiometer in addition to each input source's pad. Such an output pot is the “MASTER” volume knob for the system.
____________________________

In many ways, this isn't very different from setting up a mixing console for a live “gig”. Each input typically has a small knob setting the assessed-gain needed to bring its level to the same 'spot' as the other inputs. The inputs are mixed together by the board's sliders (simplistic, but lets keep it that way!), and the mixed material is channeled off to typically 2 to 4 outputs. Each of which has its own “master slider”. That way, left-and-right balance can be clearly adjusted, as well as mid-venue or musician-facing monitor levels. It is exactly the same.

I apologize profusely… for kind-of hijacking this thread with the above. Yet, if it is rolled “over the mind” a few times, I think y'all will see the usefulness of the analogy, in advice given to the OP for his preamp-question.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The NAC152 clone kit isn't a universal preamp for all amplifiers. Neither are similar Chinese Naim clone kits and finished preamps that I've had a look at. I've noticed that the kitsellers or perhaps the board assemblers, also make errors, fit wrong component values and don't match semis etc. in the way Naim have done with their audio electronics, right from the beginning. YMMV

Also, Naim's specified interconnection wiring is unique and intended for an outboard power supply with an earthing arrangement where there is a single earthing point at the signal input. That is necessary in the original Naim products, so it doesn't surprise me that something sounds wrong when you couple your clone to any other amplifier that likely is intended for a different earthing arrangement. The assembled clones have an internal power supply, so there's a significant difference and a noise issue already.
 
My bets would be on part value or assembly issues as well. iPod max output tends to be at ~1.2 Vrms - if this circuit can't reach that without audible issues, something is seriously amiss. (Now many power amps reach full power at somewhere from 0.75 to 1.5 Vrms, so a dedicated preamp is in fact not strictly necessary, except maybe for noise considerations, but even that is not at all a given.)

I would bust out the schematic and visually inspect the circuit to ensure that everything is where it should be and the correct value (and orientation). Suffice it to say that things won't work very well if pnp and npn transistors have been swapped, or the 27R power filter resistor has been equipped as a 330k instead. What kind of power supply is being used? Some verification of voltages with the multimeter may prove helpful, up to the point of measuring voltage on every transistor leg and posting it here alongside the schere.

Bit of a weird circuit. Modest open-loop gain and less than extreme PSRR in general, so needs a reasonably clean supply. Compensation on the output follower. I've seen kit schematics where the gain setting resistors incorrectly are 12k/1k instead of the original 12k/2k2. I see some tweak potential.
 

Jrp27

Member
2016-04-22 7:27 pm
I don't have the schematic. I just got the bare PCB and fitted the parts according to the markings. Here is the product link -

AliExpress

The board uses 2n5089 transistors maybe I can replace it with better low noise and high gain transistors like MPSA12 or so.

For grounding I used a star single point ground which is further connected to a 3 pin socket and then to the ground via mains socket.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The preamps PCBs are in 2 parts - first a gain stage then followed by a time-aligned filter as your attached schematics copied from acoustica.org suggest. Why not investigate further? You could listen to the output of the gain stage alone to narrow down where you perceive the problem.

The 2N5089 and 2N5087 that have been fitted, are used in a lot of recent Naim clones without problems. Unless they are dodgy brands or look like fakes rather than genuine product, I would not worry about them as they are good low-noise parts. The semis are not particularly critical here anyway.
 
Last edited:

Jrp27

Member
2016-04-22 7:27 pm
The preamps PCBs are in 2 parts - first a gain stage then followed by a time-aligned filter as your attached schematics copied from acoustica.org suggest. Why not investigate further? You could listen to the output of the gain stage alone to narrow down where you perceive the problem.

The 2N5089 and 2N5087 that have been fitted, are used in a lot of recent Naim clones without problems. Unless they are dodgy brands or look like fakes rather than genuine product, I would not worry about them as they are good low-noise parts. The semis are not particularly critical here anyway.

Thankfully i had scanned the blank PCB. Here is the image:
[IMGDEAD]http://convomax.com/nac152.jpg[/IMGDEAD]

Can you tell me what i need to probe and which points do i need to listen to.