RSP-1068 output level/impedance causing hiss?

[whoaru99]

As mentioned in a previous thread, I purchased a 1068 last weekend and am still in the learning mode...

I've noticed that I need to adjust the 1068 volume control fairly high to get the sound levels I like. The problem is that I seem to get some hiss on the quieter passages if I've selected modes other than 2-ch bypass.

In comparing the 1068 output to my amp input specs, I find that the 1068 is spec'ed at 1 volt / 1K ohm out but my amps are spec'ed at 1.55 volts / 20K ohms in. Is the 1.55 volt for the amps to cause full output? My old preamp is listed as 1.5 volts / 47 ohms for the output.

Is it that the output level and or impedance of the 1068 are incompatible to properly drive the amps and that is what is causing my need to turn the volume control so high and subsequent hiss through some type of gain problem?

Thanks!

[DrBoom]

I've emailed Rotel about that output voltage level too, my 1098 has an output level of 1.2V according to the site, but only 1.0V according to the manual, so which is it ?
My Parasound poweramp has an input sensitivity of 1.5V for maximum output.
Regarding the output impedance of the 1068, if it's really 1 kOhm as you say it is, that's very high.
Normal output impedance for a preamp is 50 ohms or less, some brands like Classé have an ouput impedance of below 1 ohm.
My 1098 is also pretty high at 250 ohms actually.
My Parasound C2 on the other hand has an output impedance of 50 ohms, and a whopping 8V output level.
And I can definately say that I'm able to play louder without distortion with the Parasound C2.
I don't know exactly how the impedance affects performance, but I do know that the hissing problem is not directly related to this.
If you browse this board, you'll find that hissing is pretty common among Rotel processor, even the 1098 has it.
The only exceptions are 2-ch bypass mode, and the multichannel inputs.
All other modes create hiss, very loud hiss even when volume goes above 70.

[Aussie Geoff]

DrBoom,

you''l find that hissing is pretty common among Rotel processor, even the 1098 has it.
The only exceptions are 2-ch bypass mode, and the multichannel inputs.
All other modes create hiss, very loud hiss even when volume goes above 70.

I have to take exception to this if it is intended to suggest that all RSP-1098s hiss above 70. I am sure this is true on your RSP-1098 but I have to say on mine (71 volume is reference @ 105dB) I get no hiss except for a very faint one on the centre channel (You have to be inches away to hear it). At 80 can can also hear a faint hiss from a foot away on the speakers - but this is inaudable further away and certainly wouldn't be heard over the 115 dB volume this would represent... Even at 90 I struggle to hear any hiss from my seating position. Are you sure there is not either something wrong with your RSP-1098 or the power supply. Certainly the whole hiss level from my system (including subwoofers) dropped markedly when I put in a high quality mains filter.

Geoff

[whoaru99]

For what it's worth towards figuring this out, I need my 1068 set at 80 to get to reference level / 105 dB.

[brucek]

whoaryou99,

Is it that the output level and or impedance of the 1068 are incompatible to properly drive the amps and that is what is causing my need to turn the volume control so high and subsequent hiss through some type of gain problem?

Input/output levels and input/output impedances are different issues in this case. Let me explain:

The typical interface between a preamp and power amp (or between a source like a CD player and a preamp) is a line level, high impedance connection, where the output impedance of the preamp is usually in the order of about 50 ohms to 600 ohms and the input impedance of a power amp is in the order of 10Kohms to 50Kohms.

This is known as a voltage bridge connection where the preamp is acting as a voltage source and almost no current is drawn. In fact, a perfect voltage source would have an output impedance of zero ohms at all frequencies. This would result in all the output voltage from the preamp being dropped across its load (the power amp) with no voltage being lost to the output impedance of the pre amplifier.

Output impedance is basically the internal resistance of an amplifier seen at its output. The value can be slightly frequency dependant because of reactance (caused by internal inductance and capacitance), but either way, a low output impedance is desirable.

In its simplest terms you can view the internal resistance of a preamplifier and its load (the power amp) as a voltage divider. The higher the output impedance of a preamplifier, the more voltage will drop across "itself" instead of its load (the power amp). This in effect means less voltage will be received at the power amp.

If the input impedance of a load device is not significantly higher than the sources impedance, the signal will be reduced or "loaded down" and the resultant signal to noise ratio and frequency response will suffer. Certainly the load can become too much for the source to supply adequately. This would be an extreme case.

Generally, a high output impedance requires close attention to cable lengths. The concern is that the high reactance (frequency dependant resistance caused by capacitance) of the longer cable, combined with a high output impedance of the preamp, creates a low pass filter which adversely affects bandwidth. This distortion of the higher frequencies increases with higher output impedances.

The interconnects capacitance results in a reactance (frequency dependant resistance) that will roll off the higher frequencies. It becomes more and more a factor, the higher the output impedance becomes. This interface is actually the same as the connection between a source like a CD and the input impedance of a preamp, so the same rules apply there too.

Anyway, there's an old rule of thumb that says, the input vs output impedance of a voltage bridge interface should be a minimum of 10:1. That's minimum. It should be greater in a high end system to ensure high frequency preservation. So, if I fed a power amp with 10Kohm input impedance I would just be OK with 1000 ohm output impedance preamp, but I certainly wouldn't want to split that signal.

Some preamps have a balanced output impedance of 600 ohms and a single ended of 300 ohms.
Anyway, 600 ohms isn't that high, in fact, it's used a lot in recording studio and broadcast equipment. Usually in consumer equipment you'll see around 100 ohms or less used for unbalanced outputs. Feed that 600 ohms to a power amp with an input impedance of 50Kohms and you can run some fairly long cables without any problem.

If you take a typically "good" interconnect with a 25pf / foot capacitance, then a 3 foot cable would realize a parallel reactance of about 106Kohms at 20Khz (worse case). This is very high and can be considered fairly insignificant in relation to a 1000 ohm output impedance and a power amps input impedance of about 25Kohms. No high frequency rolloff will result.
But, if I used the same cable and ran it 50 feet, then at 20Khz the cable would be introducing a reactance of about 6.3Kohms. You now have to start watching your output impedance. You can do the voltage divider math.

This reasoning is why they insist when you use a "passive" preamp you use super short, super low capacitance cables, because they have fairly high output impedance's. This impedance usually varies too, because the output is connected directly to a potentiometer that alters the output impedance as the dial is turned - pretty undesireble. People weigh this off against the benefit of low noise of the passive preamp.

So, is 1000 ohms output impedance out of line really? It's high, but any reasonable length cables won't be interacting with that impedance. I don't feel this is a contributing factor to your hiss problem.......

Anyway, if I look on the Rotel site at the specs for the 1068 for S/N ratio, I see that they are A-weighted (not unusual) and that in analog bypass the spec is 95dB and for digital at full scale it is 92dB. A-weighting of course produces a much better spec. This is basically a brick wall filter at 22khz that they use when taking the spec. This will affect THD+N numbers and S/N numbers quite a bit (in their favour) since the harmonics of that noise that they've filtered will reach into the audible range under 20Khz. So the 92dB spec for digital is likely even lower in reality.

A CD with 16 bits has a dynamic range of about 98dB. Hopefully to resolve all 16 bits the system will have a signal to noise ratio of this amount. Here, your 92 dB resolves about 15 bits - not bad, but this is where your noise is likely coming from in digital mode, specifically since you also appear to have a bit of a voltage mismatch between your preamp and amp which exacerbates the situation.

A one volt maximum output from your preamp doesn't seem very high (I'll take your word that this is max out and not nominal level). If we stick to the standard dBV reference, then that's 0dBV maximum output from your preamp. Consumer equipment generally will use a nominal output level of -10 dBV (0.316 volts rms), but to ensure decent headroom and power amp compatability the actual maximum signal voltages should reach somewhere between +4 to +10 dBV, which corresponds to about 1.6 to 3.16 volts rms maximum output (many preamps will output more - my Bryston has a 8Vrms output in digital and 10vrms in analog). You want to ensure you can drive an amplifier to full power while keeping the volume level reasonable.

The amplifier input sensitivity spec is generally the signal level required to obtain full power at the amplifier's output. This is determined by the gain and power rating of the amp. If your power amp specifies 1.55 volts, and the max output of your preamp is 1.0volts (with 92dB S/N) then your preamp will need to be turned up higher to drive that power amp, exposing the preamps noise floor. This is what's likely happening. Combine that with speakers that may need to be driven harder, you can end up with some hiss - it all adds up.....

Anyway, those are just some thoughts on it. :roll:

brucek

[whoaru99]

Thank you for the detailed explanation, I found it quite interesting and helpful.

The input/output voltage mismatch and noise floor thing does seem to explain the hiss at higher settings of the volume control.

Upon further experimenting I have found that the hiss virtually goes away (unless you put your ear right up to the speakers) once the processor receives a Dolby Digital signal from my DVD player. The hiss stays away even if I take the DVD out and go to the processor and change surround modes. The hiss then returns if I change input sources, i.e. change from Video 2 (DVD) back to CD (stereo), as before the hiss also goes away in bypass mode.

What would cause the hiss to disappear when the unit "locks" on to a digital signal even though there is seemingly no program material going to the processor input? Is this something that possibly could be fixed with a software update?

[brucek]

What would cause the hiss to disappear when the unit "locks" on to a digital signal

On most processors when a digital source is selected they will mute their volume controllers and begin monitoring the input to try and detect the manner of SPDIF bitstream that you're sending it. It could be PCM, AC3 or DTS. When the DSP's detect a recognizable bitstream they select the correct decoding and switching routines and away they go.

Of course in analog bypass, none of this is necessary, but in digital the unit has no way of knowing what you'll send it until the bitstream arrives.

Sounds like your unit works fine once locked on, but upon initial switching sources before it gets the first bitstream, your volume isn't completely muted.....

Just a guess of course, who knows, maybe it's supposed to work that way. Ask someone else around the forum to test their same processor and see if a similar situation occurs... If not, you may have a fault. If it's repeatable and consistent, it can be fixed....or you could live with it.

brucek