Can CD input be used as line level in?

[newey]

Quick question:

As with many amps nowadays, my Fender has 2 RCA jacks for plugging in a CD player, tape deck, etc. Since the amp is mono, these obviously get combined into one mono signal inside the amp. And I assume these inputs bypass the preamp, correct?

Could I use a "Y" patch cord, with 1/4"-to-dual RCA plugs, to plug in a guitar preamp or effects unit, using the line out on the preamp? In other words, are the RCA jacks equivalent to a "line level in" on the amp?

I assume this would be OK to do. But I don't want to risk any damage to the equipment at either end. The manual for the amp just says "Plug in a CD player to play along!" but doesn't give an input impedance spec for the RCA inputs, or any other info for that matter.

Would I need to use a "Y" adapter into both RCA jacks, or would one channel be sufficient, input impedance-wise?

Just wanted to check before I blow up the basement . . .

[ChrisK]

What amp is it? We could look for schematics.

There should be some specifications on CD players for the RCA phono jack outputs such as maximum level, output impedance, etc. This could give a clue.

[sumgai]

newey,

Most of the time, when they say "Plug in your Ipod or CD player", they're looking for an signal strong enough to drive headphones to decent volume levels. The nominal impedance of most headphones is 32Ω, so I'd expect that the input impedance of those RCA jacks to be similar.

Now then.....

Line Out outputs differ in two significant ways: they're much higher in impedance, and their signal level is much lower. That sort of mismatch pretty much sounds the death knell for "auto sensing and switching" between one or the other on a single pair of RCA inputs. Absent a manual switching scheme, then the label is probably gonna preclude using the "opposing" kind of input.

However.....

The output of most effects units usually isn't strong enough to drive headphones, but some of them indeed can do so. There's no reason at all not to try it! If your unit can produce decent volume in a pair of headphones, then I'd say it'd probably do OK with the RCA inputs on your amp.

As to your Y cable.....

If your effects unit does have the requisite oomph, and it's mono, then you don't need a Y cable, either of the two RCA inputs will do just fine. If your effects unit can do stereo, and you normally take advantage of that feature, then you'll probably benefit from a Y cable - but no guarantees.

From what you've described, I'd say that experimentation would be the order of the day. - you can't hurt anything.

HTH




sumgai

[newey]

ChrisK asked:

What amp is it? We could look for schematics.

It's a Fender Frontman 25R. The Fender website's schematic dates to Feb. 2000, at a time when this amp apparently featured an "Ext Sp" jack in the place my amp has the 2 RCA "Aux In" jacks.

I wish mine did have the Ext speaker jack, that'd be a lot more useful to me. But it seems that, while the amp changed, Fender's schematic didn't.

Granted, these are made for Fender in Indonesia, but you'd think they would update their website at some point- I've had the amp for about 4 years now.

My stereo's CD player lists "Output level 2 volts r.m.s."

Sumgai said:

There's no reason at all not to try it! If your unit can produce decent volume in a pair of headphones, then I'd say it'd probably do OK with the RCA inputs on your amp.

With your consent I can experiment further still . . .

BTW, the manual does indicate that the RCA inputs are not affected by the amp's V & T controls, any input must be controlled at the source. Which confirms that these must go direct to the power section (There is no Master Volume on the amp). This would limit trying this somewhat, since there's no tone stack on my effects unit. My BBE acoustic pre has a parametric EQ and a notch filter, though, so that would give some control. Both units have an output gain control.

Line Out outputs differ in two significant ways: they're much higher in impedance, and their signal level is much lower. That sort of mismatch pretty much sounds the death knell for "auto sensing and switching" between one or the other on a single pair of RCA inputs. Absent a manual switching scheme, then the label is probably gonna preclude using the "opposing" kind of input.

This is interesting since the amp manual does indicate that the "Headphone" jack can be used as a Line Out with either a TRS or TS jack (it also accepts either mono or stereo 'phones).

[ChrisK]

Here's the manual from Fender support if you need it.

www.fender.com/support/manuals/pdfs/manuals_elec/guitarpdf/Frontman_25R.pdf


Aha, he chortles; here's a schematic from the web site that shows the RCA input jacks.

www.fender.com/support/amp_schematics/pdfs/Frontman_25B_Schematic_5CE.pdf

Note that J5 has each channel going thru a 68K resistor to a common 0.1 uF capacitor to the virtual ground (- input) on U2B pin 6. This node is forced to act like a virtual ground point (signal reference, not the gameroom carpet) as the OP AMP will do everything in its power to keep the (-) input at the same voltage level as the (+) input (pin 5), which is connected to signal ground. This is a minimum of 68K Ohms of input impedance.


This schematic tells you a lot about the effects loop (J3/J4).


Keep both of your shoes on and don't soak the gameroom carpet in beer until you've got it working to your satisfaction.


And remember, beer is only "rented". ;D

[newey]

Aha, he chortles; here's a schematic from the web site that shows the RCA input jacks.

www.fender.com/support/amp_schematics/pdfs/Frontman_25B_Schematic_5CE.pdf

Look again, Chris- That's a Frontman 25B, which is the bass version of this amp (two inputs (presumably Hi/low), no gain channel, no reverb), and where I have the 2 RCA jacks, that one has a "preamp out" and "power amp in" which look to be 1/4" jacks, not the RCA ones. So I don't see how that tells me much . . .

Mine also does not have an effects loop. So far as I can tell, the schematic I linked to is the one for my amp, except for the substitution of the external speaker jack for the "Aux In" RCA jacks that I have.

[sumgai]

newey

You forget - Chris thinks in terms of modules (when he's not thinking in terms of vectors ). Dollars to donuts, you can safely cut out the bass amp's module regarding the RCA jacks, and insert it directly where it would be found in your not-quite-correct Frontman schematic.

HTH



sumgai

[newey]

OK, now I see them on there, they are apparently on the rear panel of the bass version. Had to magnify that pdf about 200% to make out the darn things!

So if we're talking 68K input impedance, that's about double what the headphones "see", meaning I should be good to go with most line-level sources, right?

Gotta make a run to the Shack for an adapter, then.

Thanks