70s SVT out to board question, which is right?

[beans-on-toast]

I wonder is the preamp out signal instrument or line level? Is there an issue with plugging a line level signal into any old DI or should it be something with a pad or speaker switch?

The genesis of this question is really more of a tangential query about amps with 1/4" line out taps. On one of my amps I have a line out that's tapped off the OT secondary with a level control, similar to the Mesa 400/+ and some others. I was doing a gig recently where the FOH didn't want to run from an unbalanced line out, and I didn't really think I needed FOH for the size of the room so I hadn't brought either of my DIs, so I ended up going DI before the amp.

The problem is, there is line level and then there is line level, there is no one standard. Consumer line level can vary and is relatively low, professional line level is +4dBu, that's 1.228V. It can be even higher, for example some preamps put out +27 dBu into 600 ohms, +32 into 10K ohms. Instrument levels also vary. A passive instrument puts out around -20dBu, an active output level can be around 10dB higher.

The power amp of the SVT requires 0.25V to drive the amp to the rated 300W power output. Clearly, this is not professional line level but is more than instrument level. This is expected because it has been through gain and conditioning stages in the preamp. I consider it a hot instrument level.

A DI should be able to accept a range of inputs. That's why DI's typically have a pad switch on the input. A common unit such as a Radial JDI, has a maximum input level of +22dBu, the pad is -15dBu. So in this case, a line level signal can go into the DI input. If it is distorted, a pad can be used.

There are inexpensive inline tip-sleeve attenuators that can be bought and used with a DI that doesn't have a pad on the input. A basic -15dBu pad like that found in the JDI is shown below, easy to build. At the bottom is a Jensen DI schematic, it shows how to use a pot to make a variable input pad. For most applications, a switch is sufficient.

With a line level off the output transformer, the maximum level can vary depending on the design. Some amps use a resistor divider to knock the level down, then use a pot to provide an adjustable level. Other amps have a transformer on the output with either tip-sleeve or balanced out.

 

[DATman]

Well thanks to everyone for the info -- TB is such a great resource for this sort of thing.
I very much appreciated reading all the perspectives and thoughts, but in summary, seems like the consensus is 1/4" instrument out to DI then XLR to board (other than mic is best, which I am getting a Sennheiser 421 which sounds like a good choice -- sorry, probably another convo). In fact, for the DI, I think I'm going to go with the Radian JDI mentioned by 73jbass (above) as I have heard good things about Radial products from several people.
Again, thanks Bass Community!

 

[beans-on-toast]

In fact, for the DI, I think I'm going to go with the Radial JDI mentioned by 73jbass (above) as I have heard good things about Radial products from several people.

Picking the right DI can be complicated!!!

Radian sells many different DI's but there are two basic types, passive and active. The tule of thumb is, if the instrument's pickups are active, use a passive DI; if the pickups are passive, use an active DI.

Why? Because by inserting a passive transformer (the JDI uses a Jensen JT-DBE, 12:1 step down) in the signal path, there is a drop in the signal level. A passive bass has a relatively low signal level to begin with. With a passive DI, you have a lower level signal going out the XLR to the mixing desk. You need to boost the level after the DI at the mixing desk preamp to make up for the loss. If the XLR cable length is shorter, this isn't as much of a problem. Longer runs require a stronger driver and receiver to go the distance with a low noise level.

An active bass has a higher signal level, the loss through a passive DI is not a problem. There is enough signal to quietly drive over a long cable. Active DI's have a buffer in them that conditions and boost the signal going out the XLR. Active DI's need to be powered, some require +48V phantom power from the mixer, some use a battery.

The two most popular Radial DI products are the passive JDI and the active J48. There are other companies that make very good DI's, Countryman, Neve (RNDI: Rupert Neve DI, check out the specs), REDDI to name a few. Radial has a page to help decide which DI to go with.

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Having said all that, I go against the rules and use a JDI with a passive bass. Works fine for me.

Take your time to decide which product best suits your needs.

 

[JimmyM]

The two most popular Radial DI products are the passive JDI and the active J48. There are other companies that make very good DI's, Countryman, Neve, REDDI to name a few. Radial has a page to help decide which DI to go with.

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While true, it should be mentioned that the REDDI won't do speaker level. It doesn't need to

Having said all that, I go against the rules and use a JDI with a passive bass. Works fine for me. Take your time to decide which product best suits your needs.

Does the passive nature of the JDI matter when you use it speaker level?

 

[beans-on-toast]

While true, it should be mentioned that the REDDI won't do speaker level. It doesn't need to

True.

Does the passive nature of the JDI matter when you use it speaker level?

That's a good question. In the case of the JDI, the speaker level signal goes through a -30 dB pad-resistor-capacitor bandpass filter that simulates the rolloff of a guitar speaker, then through the -15 dB input pad, then through the primary of the Jensen JT-DBE transformer. So 45 dB of power attenuation not counting the loss associated with the transformer. The transformer has a 12:1 step down to take the signal to mic level.

The whole purpose of the DI is to convert the high impedance instrument signal to balanced low impedance signals that can drive long cables running to the mixing desk. There has to be a low transformer output impedance and enough drive to get the signals from the DI to the mixing desk without increasing noise, adding distortion, etc.

The speaker signal level can be hotter than that of an instrument. The output power of the amp depends on how loud it is turned up. So it isn't a given that the signal level is aided by using the speaker level tap.

 

[JimmyM]

That's a good question. In the case of the JDI, the speaker level signal goes through a -30 dB pad-resistor-capacitor bandpass filter that simulates the rolloff of a guitar speaker, then through the -15 dB input pad, then through the primary of the Jensen JT-DBE transformer. So 45 dB of power attenuation not counting the loss associated with the transformer. The transformer has a 12:1 step down to take the signal to mic level.

Important is the current drive capability. The whole purpose of the DI is to convert the high impedance instrument signal to balanced low impedance signals that can drive long cables running to the mixing desk. There has to be enough drive to get the signals from the DI to the mixing desk without increasing noise, adding distortion, etc.

The speaker signal level can be hotter than that of an instrument. The output power of the amp depends on how loud it is turned up. So it isn't a given that the signal level is aided by using the speaker level tap.

Hmmm...so maybe you'd want something else for bass instead of the JDI? Does it cut lots of lows? Always was my beef with those speaker sims on guitar DI's like the Red Box. Too many lows get cut for bass.

 

[two fingers]

Speaker level DI and done.

 

[beans-on-toast]

Hmmm...so maybe you'd want something else for bass instead of the JDI? Does it cut lots of lows? Always was my beef with those speaker sims on guitar DI's like the Red Box. Too many lows get cut for bass.

I know what you mean, the idea of a 12" guitar speaker sim circuit doesn't sit well with me. They say that it is a bandpass filter so it does cut the lows and highs. And what about the harmonics that are lost when the highs are cut. If you want to capture the mid grind it isn't a problem but it won't provide hi-fidelity. The thing is, they conveniently don't provide any information on what the bandpass high and low rolloffs are.

I like the Jensen transformer, shielding, and build quality, that's why I went with the JDI. I figure that I can get around any limitations imposed by the sim. It works alright as is for my needs but my intent is to look at the circuit with some test equipment, do the math, and see what is going on, then modify it. Perhaps, rather than a bandpass, convert it to just a pad. Haven't had the time to get to it and time keeps slipping away.

They promote the speaker sim as a feature. Thinking about it, by bandpassing, they reduce the energy, allowing the unit to handle more power. From what I can tell, other DI's such as the Countryman Type 85 and the Neve RNDI do not bandpass. Countryman specifically says that they don't. This is a plus.

 

[isher1992]

Get a passive DI that can handle speaker input. ART makes a few (I am experimenting with one), I'm sure Radial does too. Just use the parallel output to your cabs. You'll have DI and mic simultaneously. That way, you'll also get the sound of your power section as well.

 

[DATman]

Based on what seems like the majority of advice, I just got a Radian JDI passive DI and appropriate cables, then also a Sennheiser 421 mic and short stand. Haven't tried it yet, but psyched to do so, and thanks everyone!

 

[coreyfyfe]

Based on what seems like the majority of advice, I just got a Radian JDI passive DI and appropriate cables, then also a Sennheiser 421 mic and short stand. Haven't tried it yet, but psyched to do so, and thanks everyone!

I dig a 421 on bass cabs, set with no roll off it keeps a solid bottom but has a nice high end. Works really well with overdrive.

 

[beans-on-toast]

When using a mic and a DI at the same time, you have to be careful to avoid phase issues. This is easy to fix in the DAW when recording. Live, if not digital, you have to optimize mic placement and distance.